The influence of environmental conditions on rates of larval growth has been documented many times for various marine mollusks. But the factors that influence rates of morphological and physiological differentiation, particularly the rate at which larvae within a population become competent to metamorphose, remain obscure. In four experiments, we reared larvae of the gastropod Crepidula plana at 29°C, 25°C, and 20°C at 30 ppt salinity, and in two other experiments, in salinities between 4-30 ppt at 25°C. Rates of shell growth and morphological differentiation, and rates of becoming competent within populations were recorded. Larvae were considered to be competent to metamorphose if they could be stimulated to metamorphose by exposure to a high concentration of KCl (20 mM above ambient). Larvae consistently became competent faster at higher temperatures, but in only one of four experiments did temperature also consistently increase the rates of growth and morphological differentiation. Larvae took longer to become competent when reared at lower salinities, but the effects were poorly predicted by the influence of salinity on rates of growth and morphological differentiation. Competent larvae could also not be recognized by shell length; many individuals were competent at shell lengths of 600-800 μm, while many other individuals were still not competent at sizes exceeding 1000 μm. At 29°C, many individuals became competent at smaller sizes than those reared at lower temperatures. Presence of gill filaments or shell brims also did not correlate with individual metamorphic competence. The data suggest that growth rate, rate of morphological differentiation, and time required for larvae of C. plana to become competent can be uncoupled markedly by shifts in rearing conditions.