Summary: 2-Deoxy-d-glucose (deGlc), d-glucosamine (GA) and l-sorbose (Sor) inhibited extension of Coprinus lagopus hyphae; deGlc was about four times more inhibitory than GA, while the latter was about five times more inhibitory than Sor. The inhibitory effects were not uniformly additive. There were indications that the analogues competed with one another, suggesting they shared at least one point of action. Acetate and fructose were virtually ineffective in reversing the inhibitory effect. Glucose was 10 times more effective than mannose and about 100 times more effective than fructose or acetate in reversing inhibition. The fact that the analogues inhibited most on media which either lacked normal carbon source or which contained acetate was interpreted as indicating that growth-rate inhibitions which occurred on media containing hexose sugar did not result from any significant effect on sugar transport. All three analogues caused gross morphological changes. The most characteristic aberration was a distinct swelling of the hyphal tip. This was most severe with GA which caused the tip cell to balloon into a spheroid some five to six times the diameter of the subtending hypha. The most extreme consequence of swelling occurred on media containing deGlc; after swelling to two to three times their normal diameters, tips frequently became vacuolated and often burst. Tip lysis was not observed on media containing either GA or Sor.