Motile cells of the unicellular green alga Chlamydomonas reinhardtii manifest chemotaxis towards sucrose, maltose, xylose, and mannitol. To obtain mutants with impaired chemotaxis towards sucrose, we used the method of random insertional mutagenesis. Strain cw15arg7 was transformed with a pARG7.8 plasmid that carried the gene of C. reinhardtii argininosuccinate lyase. Out of 68 transformants screened for chemotaxis, strain TF1, which had completely lost chemotaxis towards sucrose but maintained the normal reaction towards other attractants, was selected. The total DNA isolated from the TF1 mutant was analyzed using Southern molecular hybridization. Tetrad analysis of crosses between TF1 and the initial strain demonstrated that the loss of chemotaxis towards sucrose resulted from the integration of the pARG7.8 plasmid into the nuclear gene controlling chemotaxis towards sucrose. Thus, nuclear transformation was shown to produce chemotactic C. reinhardtii mutants.