Slipped-strand mispairing (SSM) may play an major role in repetitive DNA sequence evolution by generating large numbers of short frameshift mutations within simple tandem repeats. Here we examine the frequency and size spectrum of frameshifts generated within poly-CA/TG sequences inserted into bacteriophage M13 in Escherichia coli hosts. The frequency of detectable frameshifts within a 40 bp tract of poly-CA/TG is greater than one percent and increases more than linearly with length, being lower by a factor of four in a 22 bp target sequence. The frequency increases more than 13-fold in mutL and mutS host cells, suggesting that a high proportion of frameshift events are normally repaired by methyl-directed mismatch repair. Of the 87 sequenced frameshifts in this study, 96% result from deletion or insertion of only or two 2 bp repeat units. The most frequent events are 2 bp deletions, 2 bp insertions, and 4 bp deletions, the relative frequencies of these events being about 18:6:1.