diff --git a/lib/poetry.js b/lib/poetry.js
index 0ddf8da7..53b521e0 100644
--- a/lib/poetry.js
+++ b/lib/poetry.js
@@ -1,15 +1,14 @@
+const fs = require('fs');
 const fse = require('fs-extra');
 const path = require('path');
 const { spawnSync } = require('child_process');
+const tomlParse = require('@iarna/toml/parse-string');
 
 /**
  * poetry install
  */
 function pyprojectTomlToRequirements() {
-  if (
-    !this.options.usePoetry ||
-    !fse.existsSync(path.join(this.servicePath, 'pyproject.toml'))
-  ) {
+  if (!this.options.usePoetry || !isPoetryProject(this.servicePath)) {
     return;
   }
 
@@ -40,4 +39,29 @@ function pyprojectTomlToRequirements() {
   );
 }
 
+/**
+ * Check if pyproject.toml file exists and is a poetry project.
+ */
+function isPoetryProject(servicePath) {
+  const pyprojectPath = path.join(servicePath, 'pyproject.toml');
+
+  if (!fse.existsSync(pyprojectPath)) {
+    return false;
+  }
+
+  const pyprojectToml = fs.readFileSync(pyprojectPath);
+  const pyproject = tomlParse(pyprojectToml);
+
+  const buildSystemReqs =
+    (pyproject['build-system'] && pyproject['build-system']['requires']) || [];
+
+  for (var i = 0; i < buildSystemReqs.length; i++) {
+    if (buildSystemReqs[i].startsWith('poetry')) {
+      return true;
+    }
+  }
+
+  return false;
+}
+
 module.exports = { pyprojectTomlToRequirements };
diff --git a/package.json b/package.json
index 1178d436..e105fead 100644
--- a/package.json
+++ b/package.json
@@ -50,6 +50,7 @@
     "tape": "*"
   },
   "dependencies": {
+    "@iarna/toml": "^2.2.3",
     "appdirectory": "^0.1.0",
     "bluebird": "^3.0.6",
     "fs-extra": "^7.0.0",
@@ -61,8 +62,8 @@
     "lodash.uniqby": "^4.0.0",
     "lodash.values": "^4.3.0",
     "rimraf": "^2.6.2",
-    "shell-quote": "^1.6.1",
-    "sha256-file": "1.0.0"
+    "sha256-file": "1.0.0",
+    "shell-quote": "^1.6.1"
   },
   "eslintConfig": {
     "extends": "eslint:recommended",
diff --git a/test.js b/test.js
index 78f60aad..55f083c1 100644
--- a/test.js
+++ b/test.js
@@ -712,6 +712,17 @@ test("pipenv py3.6 doesn't package bottle with noDeploy option", t => {
   t.end();
 });
 
+test('non build pyproject.toml uses requirements.txt', t => {
+  process.chdir('tests/non_build_pyproject');
+  const path = npm(['pack', '../..']);
+  npm(['i', path]);
+  sls(['package']);
+  const zipfiles = listZipFiles('.serverless/sls-py-req-test.zip');
+  t.true(zipfiles.includes(`flask${sep}__init__.py`), 'flask is packaged');
+  t.false(zipfiles.includes(`boto3${sep}__init__.py`), 'boto3 is NOT packaged');
+  t.end();
+});
+
 test('poetry py3.6 can package flask with default options', t => {
   process.chdir('tests/poetry');
   const path = npm(['pack', '../..']);
diff --git a/tests/non_build_pyproject/.gitignore b/tests/non_build_pyproject/.gitignore
new file mode 100644
index 00000000..3c2369dc
--- /dev/null
+++ b/tests/non_build_pyproject/.gitignore
@@ -0,0 +1,22 @@
+# Distribution / packaging
+.Python
+env/
+build/
+develop-eggs/
+dist/
+downloads/
+eggs/
+.eggs/
+lib/
+lib64/
+parts/
+sdist/
+var/
+*.egg-info/
+.installed.cfg
+*.egg
+
+# Serverless
+.serverless
+.requirements
+unzip_requirements.py
diff --git a/tests/non_build_pyproject/handler.py b/tests/non_build_pyproject/handler.py
new file mode 100644
index 00000000..5e2e67ff
--- /dev/null
+++ b/tests/non_build_pyproject/handler.py
@@ -0,0 +1,5 @@
+import requests
+
+
+def hello(event, context):
+    return requests.get('https://httpbin.org/get').json()
diff --git a/tests/non_build_pyproject/package.json b/tests/non_build_pyproject/package.json
new file mode 100644
index 00000000..d13fd651
--- /dev/null
+++ b/tests/non_build_pyproject/package.json
@@ -0,0 +1,14 @@
+{
+  "name": "example",
+  "version": "1.0.0",
+  "description": "",
+  "main": "index.js",
+  "scripts": {
+    "test": "echo \"Error: no test specified\" && exit 1"
+  },
+  "author": "",
+  "license": "ISC",
+  "dependencies": {
+    "serverless-python-requirements": "file:serverless-python-requirements-4.2.5.tgz"
+  }
+}
diff --git a/tests/non_build_pyproject/pyproject.toml b/tests/non_build_pyproject/pyproject.toml
new file mode 100644
index 00000000..41932632
--- /dev/null
+++ b/tests/non_build_pyproject/pyproject.toml
@@ -0,0 +1,10 @@
+[tool.black]
+line-length = 79
+py36 = true
+skip-string-normalization = true
+exclude = '''
+/(
+    \.serverless
+    | node_modules
+)/
+'''
diff --git a/tests/non_build_pyproject/requirements.txt b/tests/non_build_pyproject/requirements.txt
new file mode 100644
index 00000000..aa55d989
--- /dev/null
+++ b/tests/non_build_pyproject/requirements.txt
@@ -0,0 +1,2 @@
+flask
+boto3
diff --git a/tests/non_build_pyproject/serverless.yml b/tests/non_build_pyproject/serverless.yml
new file mode 100644
index 00000000..973132c8
--- /dev/null
+++ b/tests/non_build_pyproject/serverless.yml
@@ -0,0 +1,21 @@
+service: sls-py-req-test
+
+provider:
+  name: aws
+  runtime: python3.6
+
+plugins:
+  - serverless-python-requirements
+custom:
+  pythonRequirements:
+    usePoetry: false
+
+package:
+  exclude:
+    - '**/*'
+  include:
+    - handler.py
+
+functions:
+  hello:
+    handler: handler.hello