The reproductive biology of the Concho water snake (Nerodia harteri paucimaculata) was studied from 1988 to 1992 in central Texas. Minimum snout-vent lengths (SVL) at maturity were 380 mm in males and 460 mm in females. Mating occurred primarily in late April and early May, although we observed some evidence of fall mating. Gestation was approximately three months. Parturition occurred predominantly in mid-August, ranging from 29 July to 22 September. Approximately 85% (range 79.5-88.8) of adult females were gravid each year with the proportion of reproductive females varying as a function of body size. Litter size averaged 11.1 (range 4-29) and varied both among and within study sites. Maternal SVL was correlated with litter size and mass but not offspring SVL or mass with litter size held constant. Litter size had no effect on offspring SVL or mass when female SVL was held constant. Relative clutch mass averaged 0.46 and was independent of female size. Mean offspring length and mass were 175 mm SVL and 3.8 g, respectively, showing no overall sexual differences. However, mean SVL was typically greater for females within litters.