Fully calcified megalospheric juveniles were found to be closely packed within a brood chamber of the larger symbiont-bearing foraminifer Peneroplis sp. (d’Orbigny) from Chuuk Island (Micronesia). The juveniles exhibit a notable variation in size of the proloculi and various forms of test deformations. Both the size variation of the megalospheric proloculi and the test deformations are indicative of asynchronous calcification and are a potential source of natural polymorphism in Peneroplis. The results of this study have implications for the interpretation of abnormal shell forms as bioindicators and place constraints on the classic definition of three size classes in Peneroplis.