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1. Many of the Ustilaginales, or smut fungi, appear to have the qualities necessary for the application of modern techniques of microbial genetics. Ustilago maydis is considered the most suitable species. 2. Investigations of the mating system confirm reports that the production of diploid brandspores in the host is controlled by alleles at two loci. 3. Genetic markers were obtained by inducing mutations in a wild-type strain with ultra-violet light. Of 100 biochemical mutants which were isolated, the growth requirements of 94 were identified. Thirty of these were used in genetic tests. 4. The compact growth of colonies on artificial media allowed new techniques to be developed by means of which large samples of progeny could be isolated and identified easily. The analysis of brandspore colonies consisting of the products of single meiotic divisions is the quickest method for detecting linkage, but its accurate measurement appears to be achieved by examining the individual members of tetrads. 5. Linkage was detected relatively rarely, but eight markers, including the a mating-type locus, were assigned to one or other of two linkage groups. Although recombination values were not always determined accurately owing to irregular basidiospore germination, the auxotrophic markers in each group could be mapped in a linear order. Since no indication of other linkage groups was obtained, the genetic evidence is so far consistent with cytological reports that the basic haploid chromosome number is two in the smut fungi. 6. Three linked markers were used to investigate chromatid interference by tetrad analysis. None was detected in a total of eighteen double exchanges.