A total of 635 DNA sequences from 35 species of mollusks were used as taxonomic support to investigate several distribution features of polymononucleotides in genomic regions of different functionality. We show that all polymononucleotide types in mollusks fit to expectations in exons but not in nonexonic regions, in agreement with a leading role of negative selection on expansions/contractions of transcription-linked poly-(A/T) repeats. The fit of all repeat length types to an exponential decay precludes the existence of a threshold size for replication slippage, a popular but unsatisfactorily explained concept in mutation models for single repeats. The genomic density of poly-(A/T) repeats is not correlated with the DNA content of species, suggesting that the differential density of repeats between species could be better explained by the species-specific performance of its repair mechanisms. This research allows a better understanding of the distribution patterns of single repeats in eukaryotes.