Nine B chromosome variants, from seven different populations of Eyprepocnemis plorans collected at four localities in Spain and three in Morocco, have been shown to be mainly composed of two DNA sequences, i.e. a 180-bp tandem repeat and ribosomal DNA. B types, however, differ in the relative amounts of the two sequences. The most widespread one (B1) bears about the same amount of rDNA and 180 bp repeat, but three other variants that have reached a polymorphism by replacing B1 in smaller areas (B2, B5 and B24) carry a conspicuously larger amount of the 180 bp repeat. In Morocco, the most widespread B variant is also B1, and a rare variant that appeared in a single individual is also built with the same two DNA sequences. All these data point to a common origin for these B chromosomes, with B1 probably being the original one. The origin of the different B types and the possible relationship of the relative amount of 180 bp DNA repeat with B drive are discussed.