During embryogenesis of the marine opisthobranch gastropod Aplysia californica Cooper, 1863, there is a brief critical time (window) during which strontium is essential for the onset of calcification. The present study was undertaken to elucidate the role of this element in mineralization. Strontium performed no structural function; deformed shells of strontium-deprived animals had normal atomic crystal structure and the element was excluded during calcification. Calcium deposition and fixation was reduced by approximately 80% in the absence of strontium but was not significantly altered in the presence of sub-optimal concentrations of this metal ion despite dramatic deficits in shell and statolith morphology. This suggests that calcium deficiency per se is not responsible for deficits induced by strontium deprivation. The reduced total calcium may be a secondary effect resulting from the complete inhibition of precipitation. Strontium did not modulate total alkaline phosphatase activity or total sulfated mucopolysaccharide synthesis during embryogenesis, and no morphological abnormalities of the organic shell were observed. Although the role of strontium in embryonic calcification of Aplysia californica remains enigmatic, these data suggest that strontium affects a highly discrete regulatory component because these more general indicators of calcification and differentiation are unaffected by its absence.