Pesticide formulations contain solvents and compounds that improve dispersion and absorption. One of the solvents employed in pesticide concentrates is xylene, an aromatic hydrocarbon widely used in paints, lacquers, or glues. This study was undertaken to evaluate the possible toxic effect of xylene on the giant cave cockroach Blaberus giganteus L. and its role in insecticide formulation toxicity. Different biochemical parameters were determined in cockroaches treated with xylene or pyrethroid formulation, in which xylene is used as a solvent. Xylene induced changes in sugar content and the level of lipid peroxidation similar to changes elicited by treatment with insecticide formulation. The present study shows that xylene induces organismal toxicity in Blaberus giganteus and should not be regarded as a carrier solvent with little biological activity. Since pesticides are always used with adjuvants that can change their toxicity, pesticide formulations should be studied as mixtures for toxic effect.