Technical Abstract: Clomazone herbicide is registered for use in watermelon; however, crop tolerance is marginal and the recommended use rates (0.07 to 0.1 kg ai ha-1) are lower for watermelon than for some other crops. In a greenhouse germplasm evaluation experiment including 56 germplasm accessions and watermelon cultivars, three Citrullus lanatus var. citroides plant introduction accessions (PI 482324, PI 5003540 and PI 532624) were not injured by clomazone; whereas, most of the other lines were moderately or severely injured. A greenhouse concentration response curve experiment demonstrated that the clomazone concentration required to cause moderate injury or growth reduction to the highly tolerant PI 500354 was approximately three to five times the concentration that caused similar injury to the susceptible PI 244017. Subsequent field experiments demonstrated that two tolerant accessions (PI 500354 and PI 482324) were injured less by clomazone and recovered more rapidly from clomazone injury than two susceptible accessions (PI 244017 and PI 271773) and two watermelon cultivars (Charleston Gray and Crimson Sweet). Tolerant germplasm lines like PI 482324 and PI 500354 may be useful as sources of clomazone tolerance in watermelon breeding.