Taxonomical and pathological studies were carried out on twenty-six strains of gram-negative bacteria isolated from epizootics in population of silkworms in various parts of Japan. The results obtained were:<br>I. These strains were classified into ten species: one strain of Proteus vulgaris HAUSER, one strain of Prateus morganii (WINSLOW et al.) RAUSS, two strains of Proteus inconstans (ORNSTEIN) SHAW et CLARKE, five strains of Serratia piscatorum (LEHMANN et NEUMANN) BREED, two strains of Serratia marcescens BIZIO (nonchromogenic), three strains of Aerobacter aerogens (KRUSE) BEIJERINCK, six strains of Aerobacter cloacae (JORDON) BERGEY et al., one strain related to Alcaligenes bookeri (FORD) BERGEY et al., one strain related to Achromobacter superficialis (JORDAN) BERGEY et al., and four strains related to Pseudomonas ovalis CHESTER.<br>II. The pathogenic effects of these strains on healthy silkworm larvae in fifth instar, which were reared aseptically on an artificial diet, were examined. The results obtained were:<br>(i) Serratia piscatorum, Serratia marcescens (nonchromogenie), Proteus vulgaris, Proteus morganii and Proteus inconstans exhibited pathogenic effects on larvae either by feeding or by injection, especially showing always 100% lethality by injection.<br>(ii) One of three strains of Aerobacter aerogens and five of six strains Aerobacter cloacae exhibited pathogenic effects on larvae only by feeding, and Achromobacter superficialis-related strain only by injection.<br>(iii) Pseudomonas ovalis-related strains and Alcaligenes bookeri-related strain exhibited little or no pathogenic effects.