Adult rabbits were immunized with nine Achromobacter xylosoxidans strains by intravenous injection of Formalin-killed organisms. Antisera thus obtained were reciprocally titrated with the nine A. xylosoxidans strains, and seven sera were defined as serologically distinct. Three of nine antisera possessed one common antibody while also each having their own specific antibody. Ninety-five strains of A. xylosoxidans were examined for serotyping by a microtiter agglutination test with the nine antisera, and the strains were divided into seven serogroups. The distribution of the 95 strains among the serogroups was as follows: serogroup A, 15 strains; B, 17 strains; C, 43 strains; D, 2 strains; E, 4 strains; G, 8 strains; F, 2 strains. Four strains were not agglutinated with any of the antisera. Serogroups A, B, and F had the ability to reduce nitrate to nitrogen gas and to grow in heart infusion broth at 41 degrees C, although there were some exceptions. In contrast, most strains of serogroups C, D, and E could not produce nitrogen gas from nitrate, although they did produce nitrite. The strains of serogroup C could not grow at 41 degrees C, whereas those of serogroups D and E could. Thus, we concluded that serogroups A, B, and F referred to biotype IIIb described by Tatum, and serogroups C, D, and E referred to biotype IIIa. The serotyping of A. xylosoxidans may be useful for the analysis of nosocomial infections caused by these organisms.