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Ultrastructural examination of deposits of reaction material (RM) showed them to be composed of paramural granules (osmiophilic to varying degrees) lying within amorphous electron–dense material. Epidermal cell walls per se became increasingly electron–dense around the short infection hyphae produced by Botrytis allii. Fluorescence microscopy, histochemical tests and microautoradiography showed the accumulation of phenolic compounds at infection sites. Use of phenylalanine and cinnamate for E.M. autoradiography demonstrated the accumulation of phenolics within paramural granules, amorphous aggregates and within the penetrated cell wall, Paramural deposits were soluble in organic solvents and probably represented reservoirs of low MW phenolics which were progressively incorporated into insoluble polymers within the cell wall. Initial synthesis or accumulation of phenolics appeared to take place in rough endoplasmic reticulum during the first 2 h after cuticle penetration. Suppression of RM deposition and wall alterations was achieved by treatment of bulb scales with cycloheximide. Tissues treated with the protein synthesis inhibitor became susceptible to colonization by B. allii. Accumulation of phenolics at infection sites rendered the onion cell walls resistant to attack by a mixture of cell wall–degrading enzymes, Methanolic extracts of epidermal strips containing RM deposits contained very weakly active inhibitors of B. allii germ–tube growth. The role of RM deposits and wall alterations in the resistance of onion epidermis to colonization by Botrytis is discussed.