The mutant strain bald-2 is unique among "flagellaless" strains of Chlamydomonas reinhardtii isolated to date, in that it possesses a mutant basal body: it is only capable of forming a ring of nine singlet microtubules, 180 nm in diameter, instead of the usual triplet basal body which is 225 nm in diameter. This singlet basal body lacks structural stability and the ability to associate with striated fiber material but retains two critical properties of basal bodies, namely, information specifying the length to which it should elongate and the ability to induce, albeit rarely, a flagellar transition region, a short, singlet-containing axoneme, and a specialized tunnel in the cell wall through which flagella normally emerge. The mutation seems to be specific for B- and C-microtubule synthesis or assembly since all other cytoplasmic sets of microtubules appear normal in numbers, orientation, and stability.