The microsporidian Encephalitozoon cuniculi can infect numerous mammals, including man. Three strains of E. cuniculi
have been identified so far, the major marker being the number of a tetranucleotide repeats in the rDNA internal transcribed spacer. We investigated diversity at the chromosomal level through the electrophoretic karyotypes obtained from 15 E.
cuniculi isolates from 5 different host species. All preparations provided patterns with 9–12 bands within a narrow
molecular size range. Six karyotype forms were distinguished, involving subdivision of strain I into 3 types (A, B, C) and
strain II into 2 types (D, E). The types A, B and C were mainly associated with isolates from rabbits of different
geographical origins. The types D, E and F were characterized by a reduced chromosome size range, 2 of these appearing
specific to a carnivorous host species (D in dog and F in blue fox). Hybridization experiments showed that all E. cuniculi
isolates possess 11 chromosomes, with a size polymorphism entailing occasional electrophoretic comigration of heterologous
chromosomes and differential migration of homologous ones. DNA rearrangements should occur during mitosis
and the hypothesis of diploidy for the basic state of E. cuniculi seems likely.