In the IsTaR 1 serodeme, we have identified variant surface glycoprotein (VSG) genes in nine different telomeric sites. We have measured the distance from the 3' end of these VSG genes to the end of the chromosome (the 'telomere length') in 20 variant antigen types (VATs) of the serodeme. Analyses of the changes in telomere length during 19 antigenic switches involving eight telomeric sites indicate a median increase in telomere length of 0.6 kilobase pairs during each switch. This may be accounted for by the 6-10 bp increase in telomere length per generation associated with DNA replication described by others. The changes in telomere lengths do not form a normal distribution since a substantial fraction show unusually large increases in telomere length or decreases in telomere length during an antigenic switch. These changes are probably caused by recombinations 3' to the VSG gene. No significant differences were detected in the behavior of telomeres at each of the eight different telomeric sites, nor were changes in telomere lengths significantly different between different antigenic switches. However, it was found that those telomeres where transcription was activated during the antigenic switch showed a significantly greater increase in telomere length than those telomeres not involved in regulation of VSG gene expression. Conversely, there was a strong correlation between transcriptional inactivation of a telomeric expression site and a decrease in telomere length. These findings suggest that processes (possibly genomic recombinations) 3' to the VSG gene coding region may be associated with a change in the transcriptional status of the VSG gene.