The paper reports the results of an investigation about the UV degradation of carbofuran, a widely used insecticide in Europe. Specific objectives were the identification of the by-products formed and the evaluation of the toxicity of the irradiated solution compared to that of carbofuran. The experimental results, obtained treating an aqueous carbofuran solution (50 mg/L) by high pressure UV lamp (125 W), show that the insecticide is completely removed within 120 min. Several intermediate by-products have been identified by liquid chromatography-mass spectrometry (LC-MS) as a result of hydroxylation process of the 2,3-dihydro benzofurane ring and other reactions such as the cleavage of the carbamate group, the hydrolysis of ethereal moiety, radical coupling and decarboxylation processes. After 270 min of reaction the identified by-products were completely degraded and COD and TOC removals of 35 and 20% were measured, respectively. Toxicological analyses performed using the comparison procedure of the MicrotoxTM assay provided interesting clues concerning toxic effects of the photodegradation by-products. The results revealed a substantial increase of the toxicity during the first 15 min proving that photodegradation of organic contaminant could even lead to an increase of the toxicity of treated solution.