Although Quinsey and colleagues (1981) argue that rapists can be distinguished from nonsexual offenders with appropriate phallometric tests, Marshall (in press) argues that the validity of such testing is not proved and that findings supportive of the testing are an artifact of population differences (i.e., psychiatric vs. correctional) and not reflective of rapists overall. This study attempted to clarify the issue by testing rapists, child molesters and mixed offenders (those with both adult and child victims) in a prison setting with both the Barbaree and Quinsey stimulus sets. Neither stimulus set distinguished the three groups in terms of the rape indexes, whereas the rape index calculated from the Quinsey stimulus set was slightly more deviant than the one calculated from the Barbaree set. However, only 25 percent of rapists were classified as deviant using a rape index cutoff of 1.0. The article discusses this finding in terms of the sexual preference hypothesis. Tables, references