Technical Abstract: Previous studies performed in our lab have measured the effect of atrazine exposure on cytochrome P450-dependent monooxygenase activity and have found increased activity in midge larvae (Chironomus tentans) as a result of atrazine exposure (1-10 ppm). Here we report the cloning and expression of a specific C. tentans CYP4 gene that is responsive to atrazine induction with an open reading frame of 1,678 bp which encodes a putative protein of 559 amino acid residues. Alignments of deduced amino acid sequences with other insect P450 genes and phylogentic analysis indicate a high degree of similarity to other insect CYP4 genes. Northern analysis employing a fragment of 1,200 bp from the CYP4 gene as a probe indicated that the CYP4 gene was expressed in all developmental stages but was expressed at highest levels in late instar larvae. Additionally, over-expression of CYP4 in C. tentans exposed to atrazine (10 ppm) confirms the ability of atrazine to induce specific P450 genes and provides insight into potential consequences of atrazine exposure in aquatic organisms.