SUMMARY: Fimbriae (pili) were demonstrated in three of four Neisseria species (Neisseria catarrhalis, N. perflava, N. subflava) by electron microscopy. Only one of three N. catarrhalis strains examined exhibited these non-flagellar appendages. Haemagglutination occurred with all strains bearing such surface structures, but different species reacted in different ways. Neisseria catarrhalis and N. perflava agglutinated mouse and rabbit red cells at 0 and 37°; N. subflava reacted similarly only with rabbit erythrocytes, and with human O cells at 0°. No reactions were observed with guinea pig, goat or sheep red cells. Prior heating of the organisms destroyed all obvious haemagglutination activity; addition of D-mannose did not inhibit haemagglutination.