Twenty-four males ofCylindrotettix obscurus and ten ofC. santarosae (Leptysminae, Acrididae) from Rio Claro, Brazil, were cytologically analysed. Both species are 2n = 23 ♂ (22+XO), all chromosomes being telocentric except for 4 acrocentric pairs ofC. obscurus. Two different B-chromosomes were found inC. obscurus: (1) a small, mitotically unstable telocentric (BT) and (2) a medium-sized, mitotically stable submetacentric (B1). B1 has a large pericentromeric heterochromatic portion and two distal euchromatic segments where both terminal associations and interstitial chiasmata are regularly formed producing a ring univalent at prophase I. B1 is almost certainly an iso-chromosome which has undergone a centromeric shift through pericentric inversion or centric transposition (heterochromatin duplication or deletion could also be the cause of arm inequality). InC. santarosae, a single male carried a small mitotically stable telocentric B-chromosome.