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Abstract Phrynops geoffroanus has a wide distribution in South America, living in aquatic environments such as rivers, streams, and lakes, in natural and urban environments. In this work, the plastron formula and the diversity index associated with the allometric condition are established for the species. For the morphometric analyses, the following were calculated: standard error, class intervals, sample standard deviation, principal component analysis, and allometry. When evaluating the morphometric dependence, r = 0.85 was obtained, which allowed us to verify the allometric classification Y, the growth rate K, and the proportion of growth b for each segment that composes the length of the plastron (LP). Thus, the calculated morphometric set demonstrates that the plastron formula can serve as a diagnostic character for Phrynops geoffroanus. We inferred from this study that the plastron formula can be maintained even in conditions of high environmental degradation.