This study was designed to evaluate whether the addition of peat moss particles to an algal diet influenced the physiology, biochemical composition, reproductive development, and survival of the Eastern oyster (Crassostrea virginica) under laboratory conditions. In two consecutive 5-wk trials (13°C to 15°C), 70-mm oysters were fed a mixed algal diet (25 μg Chl L⁻¹ or 2.3 mg TPM L⁻¹) supplemented with 3 concentrations of peat particles (2.7, 11.2, or 31.2 mg TPM L⁻¹). The proportion of organic matter (POM:TPM) in the various diets increased from 79% for the algal control to 96% for the highest peat concentration. Following a 5-d acclimation period, oysters in the various treatments exhibited similar clearance rates calculated either on the basis of chlorophyll (2.3 L h⁻¹g⁻¹) or particle removal (2.8 L h⁻¹g⁻¹). Chlorophyll filtration rates were similar among treatments (63 μg Chl h⁻¹g⁻¹), whereas particle filtration rates increased with peat concentration in the diet (9–58 mg TPM h⁻¹ g⁻¹). Although pseudofeces production rates increased with peat concentration (2–30 mg TPM h⁻¹g⁻¹), estimates of particle ingestion rate also increased (7–28 mg TPM h⁻¹g⁻¹). Fecal production rates were similar for the various treatments (2 mg TPM h⁻¹g⁻¹) as were respiration rates (0.32 ml O₂ h⁻¹g⁻¹). Oysters sampled after 3-wk and 5-wk exposure to the four diet treatments exhibited no significant differences in biochemical composition, gonad maturation rate or survival. In general, this study suggested that the presence of peat particles in an excess algal diet had no impact on oyster performance. However, the results of these laboratory trials may not be directly applicable to oysters subjected to similar peat concentrations under field conditions.