We evaluated, for brook troutSalvelinus fontinalisin three streams that undergo episodic acidification during critical periods of embryo development, survival of embryos from egg deposition to preemergence in natural redds and survival of sac fry in toxicity tests done in situ. Twenty-five natural redds were used for comparisons among streams. Median survival to preemergence (range, 16–68%) was different (P≤ 0.05) among streams and was inversely related to stream concentration of inorganic monomeric Al. Survival to preemergence was not related to intragravel dissolved oxygen concentration, gravel quality, or depth or velocity of stream water at redd sites. Median survival of sac fry exposed to stream water for 39 d was different among streams (range, 51–95%) and was inversely related to stream concentration of inorganic monomeric Al. Episodic acidification could lead to declines in populations of brook trout by causing decreased survival of early life stages.