1. 1. Rabbit antisera against the radular muscle myoglobins of the gastropod molluscs Busycon caricum and Littorina littorea were used to study immunological cross-reactivity between these and other prosobranch myoglobins. 2. 2. Qualitative and quantitative immunochemical analyses demonstrated cross-reactivity between myoglobins of the congeneric neogastropod species B. caricum and B. canaliculatum, but not between these myoglobins and those of the mesogastropod Littorina littorea. 3. 3. The results of similar studies also showed a lack of cross-reactivity between the monomeric myoglobin of the mesogastropod Polinices duplicatus and the myoglobins of Littorina littorea and Busycon caricum. 4. 4. B. caricum and B. canaliculatum myoglobins demonstrated 74 and 96% cross-reactivity as measured by the quantitative precipitin reaction, and the results of micro-complement fixation tests indicate that these proteins differ in primary structure by about 6–8%. 5. 5. The minimum mutation rate for the Busycon myoglobins, based upon their immunological cross-reactivity, differences in amino acid composition and the fossil record, was estimated to be about 12 acceptable amino acid substitutions per 100 million years.