A new sensitive analytical procedure for the determination of a N-methylcarbamate pesticide (carbaryl) is presented, based on the participation of a fluorescent derivative of the pesticide in the peroxyoxalate chemiluminescent (PO–CL) system using imidazole as a catalyst. The chemiluminescent emission is detected in a flow injection (FI) manifold, employing sodium dodecyl sulfate micellar medium as carrier, which avoids the rapid degradation of a PO in water. The formation of the fluorescent derivative implies the previous decomposition of carbaryl to methylamine (MA) by UV irradiation and subsequent derivatisation with o-phthalaldehyde (OPA), carried out off-line. Photodecomposition allows a rapid degradation of carbaryl in MA, which implies some advantages in relation to the traditional hydrolysis in alkaline medium, such shorter analysis time and easier application. The optimisation of the operational and chemical variables affecting the CL response was carried out by using experimental design strategies. The method has been successfully applied to different natural waters and vegetal sample, obtaining satisfactory recoveries in both cases, with detection limits in compliance with the maximum limit of residues (MLR) permitted.