The filtration rate of gravid females is signifiantly less than that of non-gravid females or males, whereas filtration rates of non-gravid females and males are similar in the wrinkle comb pearl mussel Cristaria plicata . Microstructure and ultrastructure of the inner and outer demibranchs of C.plicata , observed by histology, SEM, and TEM, show that a number of longitudinally arranged branchial filaments on each demibranch are connected by interfilamentary junctions in the intermediary zone, and by interlamellar septa in the lateral zone. The interlamellar septum of the inner demibranch of the female was about 2 to 3 times larger than those of the outer demibranch. The outer demibranches of gravid mussels are observably broader than those in non-gravid female and male mussels. There are no significant differences between sexes or gravidity in the distribution of the frontal, laterofrontal, and lateral cilia on the surface of the branchial filaments. Rows of interfilament canals, 58-85 μm in major axis, are located between adjacent filaments. The ultrastructure of the protuberances and microvilli of the gill epidermal cells, observed through TEM, is consistent with previous observations on other Unionidae species. Ultimately, differences in the structure of the outer demibranch of gravid mussels, especially modification of primary water tube and appearance of secondary water tube, increasing folds on the surface of filament epithelium appear to affect their filtration rate.