The greenback cutthroat trout Oncorhynchus clarki stomias is a threatened subspecies native to the upper South Platte and Arkansas rivers between Denver and Fort Collins, Colorado, an area also susceptible to acid deposition. In laboratory studies, we exposed this subspecies to nominal pHs of 4.5–6.5 and to nominal aluminum concentrations of 0, 50, 100, and 300 μg/L; the control was pH 6.5 treatment without Al. We used soft water that contained 1.3 mg Ca/L. Exposures of 7 d each were made for four early life stages: fertilized egg, eyed embryo, alevin, and swim-up larva. Effects were measured at the end of exposure and again after a recovery period lasting until 40 d posthatch. The alevin stage was the most sensitive: at pH 5.0 with no Al, survival was reduced by 68% and swimming duration by 76%; at pH 6.0 and 50 μg Al/L, swimming duration was reduced by 62%, but survival was not affected. Reductions in whole-body concentrations of Na, K, and Ca indicated organism stress. Sodium was reduced most—about 50% in alevins exposed to pH 5.0 without Al and to pH 6.0 with 50 μg Al/L. Growth and the ratio of RNA to DNA were not affected by any exposure. All responses that were affected during exposure returned to normal by 40 d posthatch. Overall, it appeared that pH 6.0 and 50 μg Al/L might be detrimental to greenback cutthroat trout populations.