Hydromedusa maximiliani is a freshwater turtle endemic to the mountainous regions of the Brazilian Atlantic rainforest. Using X-rays techniques, we provide the first substantial records of the species' reproductive biology in the wild. Twenty-six of 42 X-rayed females from 2007 to 2009 were ovigerous between September and December and 9 had eggs each year during the 3-yr period. Clutch size varied from 1 to 3, with 55% of females having 2 eggs. The smallest reproductive female was 100.2 mm in plastron length, corresponding to an age of about 13 yrs old. Significant positive correlation was verified between female body size and both egg size and clutch volume. Nesting was inferred to occur from late spring to early summer (November and December). These findings are important for management programs since this vulnerable species inhabits a threatened biodiversity hotspot.