The filtration and respiration rates of various size classes (35-39.99, 40-44.99,45-49.99, 50-54.99 and 55-59.99 mm) of the short-necked clam Paphia undulata were measured in the laboratory. The effects of three light regimes (0 lux, 172.22 lux and 645.83 lux), three microalgal species (Isochrysis galbana, Tetraselmis tetrahele and Chaetoceros calcitrans) and four microalgal concentrations (10, 25, 50 and 100 x 104 cells ml-1) on filtration rates were investigated. Mean filtration rate was highest (0.57 ± 0.04 Lh-1ind.-1) under total darkness. This can be attributed to the natural environment of this species which is characterized by silty substrate and low visibility. Filtration was also highest in the microalga Isochrysis galbana (0.67 ± 0.05). Rates initially increased from low to moderate microalgal concentrations (25 x 104 cells ml-1) and decreased at higher concentrations. Filtration generally decreased with increase in clam size. Light intensity, microalgal species and microalgal concentration showed significant effects on filtration. Respiration of fed clams was higher (0.138 ± 0.026 ml O2h-1ind.-1) than unfed clams (0.053± 0.025 ml O2h-1 nd.-1) and increased with clam size.