Recent studies of ambulatory behaviour indicate that people follow a curvilinear path when moving toward a target and wearing a prism. These results have been interpreted to indicate that heading judgments are based on moving toward a target not on optic flow. This interpretation assumes that if heading judgments were based on optic flow information they would not be altered by prism alignment. We tested this assumption. Thirteen observers made heading judgments using a relatively standard optic flow display. Observers were seated 50 cm from a display that subtended 44.6 arc deg in the horizontal dimension. The observers' task was to indicate if the focus of expansion (FoE) was to the right or left of centre. A double staircase was employed to determine right and left thresholds for the FoEs. Bias was the mean of the right and left thresholds. Precision was 1=2 the difference of the right and left thresholds. Observers repeated the judgments 3 times under 3 conditions: no prism and 2 prism conditions (15 diopters binocular, yoked base right and left). A 3 Â 3 Latin square design was used to avoid order effects. Wearing prisms had a statistically significant effect on subjects' bias ( p < 0:05) without a statistically significant effect on their precision ( p > 0.10). As prisms changed from 15 base left to 15 base right there was a shift to the right in the bias of observers' judgments of about 38 arc min ( p < 0.05). 11 of the 13 observers showed this change. The precision of judgments of heading was unaffected. The difference between the precision of base left and right judgments averaged about 6 arc min. Prism orientation affects the bias of heading judgments in optic flow fields. Precision of heading judgments is unaffected.