a b s t r a c tIn the present study, some biological traits of Caridina fossarum reproduction are studied in the GhompAtashkedeh spring (Fasa, Shiraz, Iran), for the first time. This Iranian endemic freshwater species was studied from April 2007 to March 2008. Minimum sexable size (MSS) and absolute or relative size at the onset of maturity (SOM and RSOM) in terms of carapace length (CL) was found to be 2.4 mm, 4.3 mm and 55%, respectively. Sex ratio showed variations on a monthly basis. Sex reversal is a possible scenario for C. fossarum because the size-specific sex ratio revealed a reverse pattern. Based on the proportions of ovigerous females, the main breeding season was determined to be from April to August, demonstrating a peak during early spring. There is a period of extended recruitment at water temperatures above 20 • C. The size at which 50% of females are mature was estimated at CL = 5.03 mm from the proportion of ovigerous females during the breeding season. The pleopodal egg number varied from 3 to 75 eggs per female with a mean value 34.39 ± 14.27. The results showed that although there is a significant relationship between carapace length CL and pleopodal egg number, it is not the most important factor affecting potential fecundity in C. fossarum due to the low values of slope (= 1.51) and determination coefficient (= 0.052).