Cyhalofop-butyl-resistant Echinochloa crus-galli (L.) Beauv. has become wide-spread, so rapid diagnosis of herbicide resistance in E. crus-galli at various growth stages is crucial for timely and effective management of herbicide resistant E. crus-galli throughout the season. This study was thus conducted to diagnose cyhalofop-butyl resistance in E. crus-galli at various stages of growth using rapid diagnostic test methods. Growth pouch, trimmed seedling, and stem node tests were conducted on E. crus-galli at seed germination, juvenile, and heading stages, respectively, and the diagnostic results were then compared with the conventional whole plant test. All rapid diagnostic tests discriminated resistant and susceptible biotypes on the basis of R/S ratios (the ratio of GR₅₀ values of resistant and susceptible biotypes) within 7 d after herbicide treatment. The statistical agreement in R/S ratios between the rapid diagnostic tests (R/S ratios, 2.0–4.8) and the whole plant test (R/S ratio, 3.6) demonstrated that the rapid diagnostic tests could be reliably applied to diagnose cyhalofop-butyl resistance in E. crus-galli at various stages of growth with significant time and cost savings compared with conventional whole plant tests. In addition, the presented diagnostic results coupled with discrimination of herbicide-resistant weeds in previous studies suggest that the rapid diagnostic tests are capable of detecting herbicide resistance regardless of herbicide in many different weed species.