the goal of this study was to better understand population characteristics of Villosa iris (Lea) (Rainbow Shell) in the Spring River drainage in north-central Arkansas through documenting seasonal spatial patterns, movement behavior, population size, size-frequency distributions, sex ratios, and fecundity. We conducted monthly mark and recapture sampling between May and September 2007 (i.e., before, during, and after spawning) and documented the sex, size, fecundity, and spatial location of individual Rainbow Shell at 2 sites (SFSR1 and SR1). Population estimates were relatively high at both sites with 166 ± 32 (SE) and 381 ± 37 (SE) individuals at SFSR1 and SR1, respectively. Sex ratio was highly skewed toward males at SFSR1 with a ratio of 1.0:2.6, but only slightly skewed at SR1 with a ratio of 1.0:1.3. Mean fecundity was 27,849 ± 11,653 (SE) and 15,089 ± 11,710 (SE) glochidia at SFSR1 and SR1, respectively. Spatially, statistically more males were found upstream of non-gravid females during the spawning period. Mean movement for all sampling events was 1.6 ± 0.53 cm/day and 1.9 ± 0.58 cm/day for SFSR1 and SR1, respectively. Home range was 29.3 ± 27.7 cm² and 43.0 ± 42.5 cm² for SFSR1 and SR1, respectively. From our study, we conclude that Rainbow Shell exhibits traits, such as male-skewed sex ratio and non-uniform distribution of males and females, that may influence fertilization rates of females.