Length variations in simple sequence tandem repeats are being given increased attention in plant genetics. Some short tandem repeats (STRs) from a few plant species, mainly those at the dinucleotide level, have been demonstrated to show polymorphisms and Mendelian inheritance. In the study reported here a search for all of the possible STRs ranging from mononucleotide up to tetranucleotide repeats was carried out on EMBL and GenBank DNA sequence databases of 3026 kb nuclear DNA and 1268 kb organelle DNA in 54 and 28 plant species (plus algae), respectively. An extreme rareness of STRs (4 STRs in 1268 kb DNA) was detected in organelle compared with nuclear DNA sequences. In nuclear DNA sequences, (AT)n sequences were the most abundant followed by (A)n · (T)n, (AG)n · (CT)n, (AAT)n · (ATT)n, (AAC)n · (GTT), (AGC)n · (GCT)n, (AAG)n · (CTT)n, (AATT)n · (TTAA)n, (AAAT)n · (ATTT)n and (AC)n · (GT)n sequences. A total of 130 STRs were found, including 49 (AT)n sequences in 31 species, giving an average of 1 STR every 23.3 kb and 1 (AT)n STR every 62 kb. An abundance comparable to that for the dinucleotide repeat was observed for the tri- and tetranucleotide repeats together. On average, there was 1 STR every 64.6 kb DNA in monocotyledons versus 1 every 21.2 kb DNA in dicotyledons. The fraction of STRs that contained G-C basepairs increased as the G+C contents went up from dicotyledons, monocotyledons to algae. While STRs of mono-, di- and tetranucleotide repeats were all located in non coding regions, 57% of the trinucleotide STRs containing G-C basepairs resided in coding regions.