Protonemata of three mosses (Bryum rubens Mitt., Campylopus introflexus (Hedw.) Brid,. and Polytrichum formosum Hedw.) were exposed in sterile cultures to low concentrations of the bracken herbicide asulam, added to the solidified growth medium. Protonemata were either exposed for 24 hours and then transferred onto media without asulam, or exposed constantly to the asulam media. Growth was measured over a three-week period as the mean of two plug diameters, and development of leafy gametophores was noted. A 24-hour exposure period to asulam had no effect on the growth or development of the mosses at concentrations of 0.001–1 g a.i. (active ingredient) l⁻¹. The growth and development of all three species was significantly affected by continuous exposure to asulam at concentrations of 0.01 g a.i. l⁻¹ and above. Dose-response curves were fitted to the data and EC₅₀ values calculated. Campylopus introflexus was the least sensitive species tested and P. formosum the most sensitive, with a 10-fold difference in sensitivity between the two. The sensitivity of B. rubens was closer to that of C. introflexus than to P. formosum. The sensitivity of mosses exposed to asulam is discussed in relation to previous laboratory experiments.