Gammarus pseudolimnaeus from two trout streams were exposed to various levels of hydrogen sulfide from 0.0007 to 0.0192 mg/liter in a flow‐through apparatus for periods of 65, 95, and 105 days in four tests. Results indicate that 0.002 mg/liter of H2S is the maximum safe concentration for Gammarus, but below this level there is increased reproduction or survival. Mean 96‐hr LC50 for Gammarus from the same populations was 0.022 mg/liter of H2S or approximately 10 times the safe level.