We used gel electrophoresis and mass spectrometry to analyze protein expression in untreated and organophosphate (OP)-challenged larvae from an OP (coumaphos)-resistant and an OP-sensitive strain of the cattle tick, Boophilus microplus. Tris and urea-soluble proteins were extracted and resolved by one-dimensional (1-D) and two-dimensional (2-D) gel electrophoresis. 1D analysis of proteins from untreated larvae revealed several strain-specific proteins, while differences in the expression of 4 proteins were observed upon exposure to coumaphos. The identities of the 4 proteins remain to be determined. Twenty proteins, abundantly expressed in both strains, were selected from 2D gels for peptide mass mapping and for peptide sequencing by matrix-assisted laser desorpŽtion/ionization time-of-flight (MALDI-TOF) and quadrupole time-of-flight (Q-TOF) tandem mass spectrometry (MS), respectively. Only one protein, tropomyosin, was unequivocally idenŽtified from its peptide mass map, while four proteins were assigned putative identities based on BLAST searching of heterologous databases with peptide sequences: a muscle-associated protein (troponin I), arginine kinase, a putative high-mobility group protein, and a small heat shock proŽtein. Three additional proteins, one of which was markedly upregulated in untreated, OP-resistant larvae, were not identifiable, suggesting they may be novel molecules. Peptide sequence information is currently being obtained for the remaining twelve proteins.