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Amphibians have long been known to display wide variation in erythrocyte morphology across species, but within species there has been little attention given to individual variation in red blood cell morphology. We captured 49 red-backed salamanders (Plethodon cinereus) from central Pennsylvania, USA and used image analysis procedures to measure erythrocyte morphology (size and shape) on blood smears made from all individuals. We then statistically examined whether variation in snout-vent-length, sex, tail loss, or capture location influenced these cell variables. Only snout-vent-length affected erythrocyte size and shape, with increasing body sizes associated with increasing cell areas and increasingly rounder cells. Further, erythrocyte shape was also associated with a measure of body condition that was corrected for body size, such that individuals with high body condition scores had rounder cells. Given the oxygen-carrying role of erythrocytes in all vertebrates, we suspect this discovery is related to size-related changes in oxygen demand, since total oxygen consumption increases with body size in an allometric manner. While our results warrant further investigation to understand the mechanism, the association we found between cell roundness and both body size and condition nevertheless indicates this parameter could be used to assess the health state of plethodontid salamanders in future research, provided non-destructive sampling is employed. Our results also underscore the value of hematological investigations in the study of animal biology. [ABSTRACT FROM AUTHOR]