Freshwater pulmonate snails (Planorbella trivolvis) were maintained in the laboratory in stream water with manipulated pH and Ca2+. Low pH treatments (4.6 and 4.7) resulted in significantly lower adult growth rates and reduced gross fecundity compared with circumneutral pH treatments (7.2 and 7.4) regardless of the Ca2+ level. None of the eggs laid in low pH treatments hatched, mostly because of developmental arrest (gastrula or earlier). Eggs in low Ca2+ treatments took longer to complete development and had a higher incidence of abnormality compared with eggs in the high pH, high Ca2+ treatment. Juveniles that successfully hatched from the low Ca2+ treatments grew very little and all died within 100 days whereas over 7.2% of those in the high Ca2+, circumneutral pH treatment were alive at day 100, some of which had become adults and were laying eggs. This study suggests that both low pH (4.6) and low Ca2+ (1–2 mg/L) are lethal to Planorbella trivolvis, with embryos and juveniles showing much greater sensitivity than adults. Given the ranges of the two variables used in this study (1000-fold for H+ and ca. 30-fold for Ca2+), pH has the greater negative impact and, when below 5.0, led to recruitment failure in P. trivolvis; a similar effect on other pulmonate species would be expected. Furthermore, a Ca2+ concentration as low as 2 mg/L alone, regardless of pH, would be sufficient to cause recruitment failure by reducing juvenile survival. These data suggest that increasing Ca2+ levels does not protect P. trivolvis from the effects of low pH.