A highly sensitive enzyme‐linked immunosorbent assay (ELISA) was developed to detect alachlor in tap water samples. When it was compared with classical detection methods such as gas chromatography/mass spectrometry, the ELISA was much faster because there were no sample preparation steps and analysis time was quicker. The average recovery over a range of fortified samples was 87% and inter‐plate variation was less than 10%. The ELISA had a sensitivity of 0.017 ppb which was five times more sensitive than the EC cut‐off of 0.1 ppb, making it suitable for the large‐scale screening of tap water samples.