diff --git a/tests/phpunit/tests/Debug/Debug_LogStringTest.php b/tests/phpunit/tests/Debug/Debug_LogStringTest.php
new file mode 100644
index 0000000..14508a6
--- /dev/null
+++ b/tests/phpunit/tests/Debug/Debug_LogStringTest.php
@@ -0,0 +1,104 @@
+<?php
+/**
+ * Class Debug_LogStringTest
+ *
+ * @package AspireUpdate
+ */
+
+/**
+ * Tests for Debug::log_string()
+ *
+ * These tests cause constants to be defined.
+ * They must run in separate processes and must not preserve global state.
+ *
+ * @runTestsInSeparateProcesses
+ * @preserveGlobalState disabled
+ *
+ * @covers \AspireUpdate\Debug::log_string
+ */
+class Debug_LogStringTest extends Debug_UnitTestCase {
+	/**
+	 * Test that nothing is written to the log file when debugging is disabled.
+	 */
+	public function test_should_not_write_to_log_file_when_debugging_is_disabled() {
+		define( 'AP_DEBUG', false );
+		define( 'AP_DEBUG_TYPES', [ 'request', 'response', 'string' ] );
+
+		AspireUpdate\Debug::log_string( 'Test log message.' );
+
+		$this->assertFileDoesNotExist( self::$log_file );
+	}
+
+	/**
+	 * Test that nothing is written to the log file when debug types are not an array.
+	 */
+	public function test_should_not_write_to_log_file_when_debug_types_are_not_an_array() {
+		define( 'AP_DEBUG', true );
+		define( 'AP_DEBUG_TYPES', 'string' );
+
+		AspireUpdate\Debug::log_string( 'Test log message.' );
+
+		$this->assertFileDoesNotExist( self::$log_file );
+	}
+
+	/**
+	 * Test that nothing is written to the log file when string debugging is disabled.
+	 */
+	public function test_should_not_write_to_log_file_when_string_debugging_is_disabled() {
+		define( 'AP_DEBUG', true );
+		define( 'AP_DEBUG_TYPES', [ 'request', 'response' ] );
+
+		AspireUpdate\Debug::log_string( 'Test log message.' );
+
+		$this->assertFileDoesNotExist( self::$log_file );
+	}
+
+	/**
+	 * Test that the message is written to the log file.
+	 *
+	 * @dataProvider data_debug_types
+	 *
+	 * @param array $debug_types An array of enabled debug types.
+	 */
+	public function test_should_write_to_log_file( $debug_types ) {
+		define( 'AP_DEBUG', true );
+		define( 'AP_DEBUG_TYPES', $debug_types );
+
+		$message = 'Test log message.';
+
+		AspireUpdate\Debug::log_string( $message );
+
+		$this->assertFileExists(
+			self::$log_file,
+			'The log file was created.'
+		);
+
+		$this->assertStringContainsString(
+			$message,
+			file_get_contents( self::$log_file ),
+			'The message was not logged.'
+		);
+	}
+
+	/**
+	 * Data provider.
+	 *
+	 * @return array[]
+	 */
+	public function data_debug_types() {
+		return [
+			'just "string"'                        => [
+				'debug_types' => [ 'string' ],
+			],
+			'"string" at the start of the array"'  => [
+				'debug_types' => [ 'string', 'request' ],
+			],
+			'"string" in the middle of the array"' => [
+				'debug_types' => [ 'request', 'string', 'response' ],
+			],
+			'"string" at the end of the array"'    => [
+				'debug_types' => [ 'request', 'response', 'string' ],
+			],
+		];
+	}
+}