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Intratest oxygen isotope variations in the planktonic foraminifer Neogloboquadrina pachyderma sinistral (left coiling) from North Atlantic core top and multi-net samples were assessed by ion microprobe analysis from 2 to 6 μm spots with precision and accuracy better than 0.7‰ in δ¹⁸O (2 SD). Within a single foraminiferal test from a core top sample comprising both ontogenetic calcite and outer crust, δ¹⁸O values vary from 0.5‰ to 3.7‰ [PDB], exceeding the range of equilibrium δ¹⁸O in the specimens' habitat by a factor of three. The isotopic difference between the ontogenetic calcite and the crust averages 1.8‰. Neither of the two types of foraminiferal calcite precipitates in equilibrium with ambient seawater. The ontogenetic calcite exhibits a negative vital effect Δ¹⁸O_(M-E) (δ¹⁸O_(measured)−δ¹⁸O_{(equilibrium)}) ranging from −0.5 to −1‰. The largest negative fractionation is associated to the inner walls of juvenile …