The distribution patterns of parasitized and nonparasitized potato aphids,Macrosiphum euphorbiae(Thomas), were assessed on four plantings of staked tomatoes from 1988 to 1990. Within plant distribution of nonparasitized aphids changed through the season, with the highest percentage of aphids found on the middle plant stratum during the first ≈60 d and on the upper stratum after this time. Aphid parasitism was inversely density dependent (i.e., rates of parasitism were highest on plantings with low overall potato aphid populations). In addition, parasitism varied within plants, with higher parasitism rates in the middle compared with the upper plant strata. Taylor's power law described an aggregated distribution pattern for both nonparasitized and parasitized aphids, regardless of leaf sample site used for analysis. Analysis of covariance for homogeneity of slopes indicated that the index of aggregation (b) from Taylor's power law remained consistent among years and among sample leaves. The third most recently expanded leaf from the uppermost of the plant (leaf T3) provided the highest relative level of precision among all single-leaf sample units for mean estimates at low aphid densities and was similar in precision to a sample unit consisting of the three uppermost leaves. A binomial sampling plan is presented that uses leaf T3 as a sample unit.