The use of N-cyclopropyl-N′-(2-fluorophenyl) urea as a selective herbicide in grain sorghum has recently been disclosed (U.S.P. 4,344,916). Evaluation of analogs of this compound has included two assays on isolated pea chloroplasts-photosynthetic electron transport and competition for atrazine binding sites. Of all the analogs studied in at least one of these assays, the most active in vitro were the N-cyclopropyl-, N-n-butyl-, and N-n-pentyl-derivatives of 2,5-difluorophenyl urea. The two in vitro assays correlated well with each other, and binding activity demonstrated a strong correlation with whole-plant phytotoxicity following postemergence application. Several postulated sorghum metabolites of N-cyclopropyl-N′-(2-fluorophenyl) urea showed weak or no activity in vitro, as would be expected from the compoud's selectivity properties.