Abstract: Fifty-eight Chinese isolates of Riemerella anatipestifer which agglutinated only in antiserum to serotype 10 representative strain (H2199) were further serotyped by agar gel precipitin methods. Based on precipitation patterns, these strains can be divided into four groups: the first group contained 30 isolates represented by C2, the second contained 3 isolates represented by C449, the third contained 6 isolates represented by C459, and the fourth contained 19 isolates represented by C598. Isolates in the first group showed identical to H2199, and isolates in other groups showed identical to representative strain of their own respectively, but had antigenic differences with H2199 or one another.Antigenic relationships between H2199 and four groups were further studied using slide and tube agglutination tests with adsorbed antiserum. Isolates represented by C2 were real serotype 10. Isolates identical to C459 showed distinct from H2199, such strains possessed their own specific antigenic materials,in addition they had antigens in common with H2199. The relationship between H2199 and C459 could be described as ab-bc. Similar antigenic relationships were observed between C598 and H2199, C598 and C449 respectively. However, compared with C449, C459 and C459 strains, H2199, C598 and C449 had no type-specific antigenic materials of their own in tube agglutination test respectively. So the relationship between H2199 and C449, C598 and C459, C449 and C459 can be described as b-bc respectively. It might be better to divide the isolates agglutinated in anti H2199 serum into subtypes 1～4 represented by strains H2199 (or C2), C449, C459, C598 respectively.