Interference of silverleaf nightshade (Solanum elaeagnifolium Cav.) with Pronto spanish peanuts (Arachis hypogaea L.) was evaluated from 1981 through 1983 in a natural occurring weed population. Treatments consisted of weed-free maintenance or weed interference for 0, 4, 8, 12 weeks and for the full season. Silverleaf nightshade stems were counted as a measure of weed regrowth in treatments maintained weed free for 0, 4, and 8 weeks. Contamination of the harvested in-shell peanuts by silverleaf nightshade berries was determined by counting the number of berries passing through the peanut combine. In-shell peanut yields were reduced by an average of 17% when silverleaf nightshade was allowed to interfere with the crop for 4 weeks. Further yield reductions of 53, 66, and 66% were observed in treatments where interference occurred for 8 and 12 weeks and for the full-season, respectively. Regression analysis conducted on yield data of individual years predicts that each week of weed-free maintenance after crop emergence results in an average of 33 to 38 kg/ha yield increase above the unweeded control. Conversely, analyses of yield data averaged over all years indicated that for each week of weed interference there would be approximately a 103 kg/ha decrease in in-shell yield compared to the weed-free control. When yield data were converted to percent of yield of weed-free controls, there was no interaction among years. Regression analysis of the converted data predicts that for each week of weed-free maintenance after crop emergence there would be a 3.7% yield increase compared to the unweeded control and that for each week of weed interference there would be a corresponding yield loss of 4.5%. Silverleaf nightshade stem counts per plot were reduced an average of 18 and 36 percent for treatments maintained weed free for 4 and 8 weeks, respectively. In 1982 analysis of fruit contamination indicated a significant difference between full-season interference and weed-free maintenance for 4 or more weeks. Differences in fruit contamination between 4, 8, and 12 weeks of weed-free maintenance were not significant. In the second year no differences in fruit contamination were observed between the weedy check and the other treatments; however, fruit production after 4 weeks of weed-free maintenance was significantly higher than after 8 and 12 weeks of weed-free maintenance. Peanut quality, disregarding contamination by silverleaf nightshade berries, was not affected by weed interference.