Two actin genes cloned from Culex pipiens (L) are upregulated during adult diapause. Though actins 1 and 2 were expressed throughout diapause, both genes were most highly expressed early in diapause. These changes in gene expression were accompanied by a conspicuous redistribution of polymerized actin that was most pronounced in the midguts of diapausing mosquitoes that were exposed to low temperature. In nondiapausing mosquitoes reared at 25°C and in diapausing mosquitoes reared at 18°C, polymerized actin was clustered at high concentrations at the intersections of the muscle fibers that form the midgut musculature. When 7-10 day-old mosquitoes were exposed to low temperature (-5°C for 12h), the polymerized actin was evenly distributed along the muscle fibers in both nondiapausing and diapausing mosquitoes. Exposure to low temperature (-5°C for 12h) in 1month-old mosquitoes elicited an even greater distribution of polymerized actin, an effect that was especially pronounced in diapausing mosquitoes. These changes in gene expression and actin distribution suggest a role for actins in enhancing survival of diapausing adults during the low temperatures of winter by fortification of the cytoskeleton.