In our everyday motor interactions with objects, we often encounter situations where the features of an object are determinate (i.e., not perceptually ambiguous), but the mapping between those features and appropriate movement patterns is indeterminate, resulting in a lack of any clear preference for one posture over another. We call this indeterminacy in stimulus-response mapping 'motor ambiguity'. Here, we use a grasping task to investigate the decision mechanisms that mediate the basic behavior of selecting one wrist posture over another in conditions of motor ambiguity. Using one of two possible wrist postures, participants grasped a dowel that was presented at various orientations. At most orientations, there was a clear preference for one wrist posture over the other. Within a small range of orientations, however, participants were variable in their posture selection due to the fact that the dowel was ambiguous with respect to the hand posture it afforded. We observed longer reaction times (RT) during 'ambiguous' trials than during the 'unambiguous' trials. In two subsequent experiments, we explored the effects of foreknowledge and trial history on the selection of wrist posture. We found that foreknowledge led to shorter RT unless the previous trial involved selecting a posture in the ambiguous region, in which case foreknowledge gave no RT advantage. These results are discussed within the context of existing models of sensorimotor decision making.