Fecundity and biomass of nine species of aphids (Hemiptera: Aphididae) feeding on six species of plants were compared to assess whether the two measures are equally effective for quantifying aphid performance. Performance was quantified by measuring both fecundity (the number of offspring born over a defined interval) and biomass (the dry mass of offspring produced) using three variables expected to affect performance: host-plant genotype, aphid genotype, and aphid density. The efficacy of the performance parameters was assessed by comparing their ability to discriminate among treatments for the three variables. Biomass usually provided a more effective measure of performance than fecundity, but for one aphid species, fecundity was more effective than biomass. Biomass of offspring is the preferred measure of performance, but biomass and fecundity should both be recorded whenever practical.