Mutants of Schizosaccharomyces pombe, which were resistant to concentrations of colcemid inhibitory to the wild-type parent, were not diminished in their permeability to colcemid or colchicine. They also did not modify colcemid to a detectable derivative or to a product that was incapable of binding to brain tubulin. The resistance of these mutants was therefore via mechanisms different from those of permeability and detoxification known for animal cells.