Studies were conducted to determine the distribution of s-triazine resistant biotypes of common lambsquarters (Chenopodium album L.) and smooth pigweed (Amaranthus hybridus L.) in Virginia. Collections of seed were made from suspected triazine-resistant biotypes of common lambsquarters and smooth pigweed from counties in Virginia which had reported having triazine resistance problems. Triazine resistance was confirmed by measuring chlorophyll fluorescence in the presence of atrazine. For further confirmation of triazine resistance in collected common lambsquarters and smooth pigweed biotypes, greenhouse testing of whole plants and a sinking leaf disc assay were performed. Cross-resistance to another s-triazine, as-triazine, and substituted urea herbicide was also determined for s-triazine-resistant biotypes. These studies have shown triazine- resistant smooth pigweed to be present in 19 counties and common lambsquarters to be present in 8 counties in Virginia. s-Triazine resistant biotypes were found to be resistant to another s-triazine and as-triazine herbicide, but were susceptible to the substituted urea herbicide. Additional studies were initiated to determine the effects of different temperature regimes on triazine-resistant and -susceptible biotypes of common lambsquarters and smooth pigweed from different geographical locations. These studies were conducted at the North Carolina State University Phytotron facility in controlled environment growth chambers. Triazine-resistant common lambsquarters biotypes from Virginia, Maryland, and Switzerland as well as a smooth pigweed biotype from Virginia were examined. Triazine-susceptible biotypes of common lambsquarters and smooth pigweed were included as controls. Shoot height, weight, chlorophyll a and b content, and whole leaf fatty acid content of common lambsquarters and smooth pigweed were determined at 18°/14° C, 26°/22° C, and 36°/26° C. Measurements of shoot height were made at 30 and 63 days after planting. The shoot weight, chlorophyll a and b content, and fatty acid content was determined from plants harvested at 63 days after planting. These data indicate common lambsquarters biotypes from different geographical regions exhibited a differential response to temperature. There was no difference between triazine-resistant and -susceptible biotypes in response to temperature. Differences were detected between triazine-resistant smooth pigweed biotypes which indicated that the susceptible biotypes were more vigorous as indicated by shoot height and weight at lower temperatures than triazine-resistant biotypes of smooth pigweed. / M.S.