The amphipod Gammarus pseudolimnaeus is abundant throughout most of the length of two small streams on the coastal plain of Virginia, but is absent from their upper reaches. The streams begin as very dilute springs but exhibit a downstream gradient of increasing dissolved ions and pH. Amphipods displaced upstream of their distribution limit in fine-mesh cages experienced rapid mortality compared to those maintained downstream of the limit, suggesting that neither interspecific competition nor predation explains the observed limit. General osmoregulatory failure during intermolt was not limiting; percent body-water content, a general gauge of osmotic stress, was not greater in amphipods closer to the distribution limit, nor did it increase in amphipods displaced above the limit. However, newly molted amphipods in water from above the distribution limit died more rapidly than those in water from below the limit. The addition of Ca ² to water from above the limit increased postmolt survival to levels observed in those maintained in water from below the limit; raising pH had no effect on postmolt mortality. Insufficient ambient calcium is a limiting factor in the distribution of G. pseudolimnaeus.