<b>Background and Objective:</b> Bamboo clam <i>Solen lamarckii</i> is one of the bivalve's species that have important economic value in Indonesian waters. Knowledge of the reproductive biology of a species is essential for proper and sustainable fisheries management. Monthly variations in the gonad maturity level of the clams from the coastal waters of Bengkalis Island Indonesia, were examined. <b>Materials and Methods:</b> Samples of the clam were collected monthly at the intertidal zone during low tide from May to September, 2020. A total of 239 specimens of which 116 females and 123 males were examined. Reproductive aspects observed in this study included sex ratio, gonadosomatic index, monthly gonadal maturity level and female fecundity. Mature female or near spawning phases were used for fecundity estimation. The sex ratios were analysed using a Chi-quadrat-Test, whereas to determine differences in fecundity between female individuals tested by Analysis of Variance. <b>Results:</b> Based on the gonadosomatic index (GSI) and histological observation of both sexes's gonads, it can be concluded that early spawning occurred in August and the peak seemed to take place during October-November. The sex ratios were analysed using a Chi-quadrat-Test which was not different from parity. Gonad maturation time between males and females indicates synchrony and no hermaphrodites were observed in the collection. Females shell length range of 75.5 to 89.3 mm produced 4.63×103-7.43×103 eggs ind-1 female (N = 10). Females with a shell length between 75.5 and 89.3 mm had varying fecundities, but analysis of variance showed no significant difference (Sig>0.05). <b>Conclusion:</b> Based on the gonadosomatic index (GSI) and histological analysis, <i>S. lamarckii</i> in Bengkalis waters spawned in August. Males and females showed spawning synchrony.
* Title and MeSH Headings from MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine.