Sperm motility, fertilization, and embryonic development were investigated in the hermaphroditic basket cockle, Clinocardium nuttallii. Motility of the sperm was more prolonged over a 2-h storage period at 4°C than at 19°C. The sperm-to-egg ratio resulting in the highest fertilization rate was 10,000:1, or an equivalent final sperm concentration of 6 × 10⁵ cells mL⁻¹, using a density of 60 eggs mL⁻¹. However, at least some degree of self-fertilization appeared to be unavoidable during the release of the eggs from the broodstock. The biological zero point, below which embryonic development ceased, was estimated to be 2.8°C to 2.9°C for C. nuttallii.