Two experiments are reported that investigate the nature of selections in the McGeorge and Burton (1990) invariant learning paradigm. McGeorge and Burton suggest that subjects implicitly acquire abstract knowledge of an invariant feature (usually the presence of the digit ''3'') in a set of 30 stimuli. McGeorge and Burton's analysis has recently been challenged by Cock, Berry, and Gaffan (1994) and by Wright and Burton (1995). In this paper, we demonstrate that performance is based on knowledge of other aspects of the learning set besides the invariant digit, but that this knowledge is still implicit. Altering the nature of the learning stimuli to highlight these co-varying features enhances the effects and increases the reporting of explicit knowledge. Our results indicate that performance within this paradigm is more easily characterized as rejection of salient negatives than selection of positive instances, but that salience is not based simply on similarity.