The constant relationship between seawater salinity and boron concentration suggests that boron can be used as a measure of salinity. Previous workers have suggested, on the basis of analyses of the boron content of shells belonging to the bivalved mollusc Mytilus edulis, that salinity could be reconstructed from biomineral records of boron. Molluscan shells grow by incremental accretion and preserve within them geochemical records of their environments. Therefore, if boron concentration is being controlled by an external factor, the ontogenetic boron profiles of different shells, contemporaneous and sympatric, should be similar. This paper reports on the analysis of three shells belonging to two species of bivalves, Chionopsis gnidia and Chione californiensis, collected from the Gulf of California. Ontogenetic profiles constructed using a spectrophotometric method are quantitatively similar among the three specimens. The range of boron concentrations in each shell coincide with estimated ambient salinity variation, but also seems to be influenced by aperiodic regional rainfall, and the resulting influx of boron-poor freshwater. The similarity among the shells demonstrates that boron concentration is under external, environmental control, and directly reflects boron concentration in the surrounding water. This conclusion raises the possibility of reconstructing ambient boron concentrations from molluscan shells, and perhaps also reconstructing local salinity values. Further research is required to determine if boron is deposited in equilibrium with ambient concentrations, the nature of any dependence of incorporation on temperature, and, therefore, if paleosalinity reconstruction is possible.