The discovery of delta-tubulin, the fourth member of the tubulin superfamily, in Chlamydomonas [1] has led to the identification in the genomes of vertebrates and protozoa of putative delta homologues and of additional tubulins, epsilon and zeta [2-4]. These discoveries raise questions concerning the functions of these novel tubulins, their interactions with microtubule arrays and microtubule-organising centres, and their evolutionary status. The sm19-1 mutation of Paramecium specifically inhibits basal body duplication [5] and causes delocalisation of gamma-tubulin, which is also required for basal body duplication [6]. We have cloned the SM19 gene by functional complementation and found that it encodes another new member of the tubulin superfamily. SM19p, provisionally called eta-tubulin (eta-tubulin), shows low sequence identity with the tubulins previously identified in Paramecium, namely, alpha [7], beta [8], gamma [6], delta (this work) and epsilon (P. Dupuis-Williams, personal communication). Phylogenetic analysis indicated that SM19p is not consistently grouped with any phylogenetic entity.