This paper compares two well-known arguments in the units of selection literature, one due to Sober and Lewontin (1982), the other due to Sober and Wilson (1998). Both arguments concern the legitimacy of "averaging" fitness values across contexts and making inferences about the level of selection on that basis. The first three sections of the paper shows that the two arguments are incompatible if taken at face value, their apparent similarity notwithstanding. If we accept Sober and Lewontin's criterion for when averaging genic fitnesses across diploid genotypes is illegitmate, we cannot accept Sober and Wilson's criterion for when averaging individual fitnesses across groups is illegitimate, and vice versa. The final section suggests a possible way of reconciling the two arguments, by invoking an ambiguity in the concept of "genic selection". © 2004 Kluwer Academic Publishers.