Studies of insect fatty acyl-CoA desaturases have heretofore concentrated on the Diptera and Lepidoptera. We report here the isolation and characterization of a fatty acyl-CoA Delta9 desaturase from the house cricket, Acheta domesticus (Orthoptera). Two desaturase cDNAs were isolated from a library, one of which contained two intron sequences. The clones were identical in their respective coding regions, but had divergent 5' and 3' untranslated regions. The cDNAs encode a 359 amino acid desaturase enzyme that could rescue a fatty acyl-CoA desaturase auxotrophic phenotype when expressed in yeast. Biochemical analysis of lipids from transformed yeast cells confirmed that the enzyme is a Delta9 desaturase with activity on both palmitic and stearic acid substrates. Southern blotting indicated multiple Delta9 desaturase genes within the genome. A single message that was up-regulated in fed insects vs. starved insects was observed on northern blots, indicating transcriptional regulation in response to diet.