Aqueous solutions of 0.01% ascorbic acid, 0.15% cysteine hydrochloride, 0.2% Bacto-Peptone, 1.0% Na2SO3, and water deoxygenated by boiling and bubbling N2, were used as stimuli to hatch eggs of 5 California strains of Aedes sierrensis. The 1.0% Na2SO3 solution produced the best mean hatch (84%) for all strains but high mortality of larvae occurred when they were left in this solution for 24 hr. By exposing eggs to 0.1% Na2SO3 for 3 hr, then diluting with fresh water, negligible larval mortality occurred and there was no change in the percent egg hatch. Considerable differences in hatching success were observed between some strains exposed to the same hatching stimuli, and between given strains exposed to different hatching stimuli.