Summary.A rapid bioassay for PEBC (S‐propyl‐N‐butyl‐N‐ethylthiolcarbamate) has been developed, based on the measurement of shoot elongation of oat seedlings in Petri dishes in a nearly vertical position. Results are obtained after 48 hr.The bioassay was used to study the relationship between soil moisture and volatilization and adsorption of PEBC. When PEBC was applied to air‐dry soil, no appreciable loss of activity occurred even after 36 days of exposure to a continuous air‐flow. The residual activity of PEBC applied to soils of various moisture contents appears to be related to the initial moisture level. A substantial amount of the compound was adsorbed by air‐dry soil after an exposure of 24 hr to vapours of PEBC. The amount of PEBC adsorbed from the vapour phase after 24 or 72 hr appears also to be related to the moisture content of the soil.Un essai biologique rapide pour U PEBC et son utilisation dans l'étude dt la volatilisation et de l'adio