Late-instar juveniles of the North American ostracode, Candona rawsoni, were cultured to maturity under controlled oxygen isotope composition and temperature (15°C and 25°C) to measure isotopic fractionation during shell calcification. The ostracode shells formed at experimental temperatures are not in isotopic equilibrium with water but had a constant offset from equilibrium based on the oxygen isotope fractionation of inorganic carbonates. The oxygen isotope composition of ostracode shells from 15°C cultures were higher by about 2‰ compared to that from 25°C cultures, a difference similar to that expected for inorganic calcite across a 10°C temperature range. The observed fractionations are expressed by the regression equations 25°C: δ18OPDB shell = -0.47 + 0.97δ18OVSMOW water 15°C: δ18OPDB shell = 1.12 + 1.07δ18OVSMOW water The fractionation factors (α) are: 1.0305 at 25°C and 1.0322 at 15°C. The shells of C. rawsoni are elevated by about 0.8-1‰ in δ18O compared to inorganic carbonates (O'Neil et al., 1969), but are close to marine biogenic aragonites (Grossman and Ku, 1986) with a slightly different temperature dependence.