We have used the PCR-based randomly amplified polymorphic DNA (RAPD) method to efficiently identify and map DNA polymorphisms in the ciliated protozoan Tetrahymena thermophila. The polymorphisms segregate as Mendelian genetic markers. A targeted screen, using DNA from pooled meiotic segregants, yielded the polymorphisms most closely linked to the mat locus. A total of 10 polymorphisms linked to the mat-Pmr segment of the left arm of micronuclear chromosome 2 have been identified. This constitutes the largest linkage group described in T. thermophila. We also provide here the first crude estimate of the frequency of meiotic recombination in the mat region, 20 kb/cM. This frequency is much higher than that observed in most other eukaryotes. Special features of Tetrahymena genetics enhanced the power of the RAPD method: the ability to obtain in a single step meiotic segregants that are whole-genome homozygotes and the availability of nullisomic strains permitting quick deletion mapping of polymorphisms to micronuclear chromosomes or chromosome segments. The RAPD method appears to provide a practical and relatively inexpensive approach to the construction of a high-resolution map of the Tetrahymena genome.