The presence of supernumerary (B) chromosomes was revealed in a sample
of the wood mouse (Apodemus sylvaticus) from the territory of the Cze
ch Republic in which the specific status was identified by investigati
on of allozyme diagnostic markers. Two individuals with one and a sing
le individual with four B chromosomes were found within a sample compr
ising 60 individuals studied both karyologically and biochemically. Th
e overall frequency of the B chromosome incidence in A. sylvaticus see
ms to be rather low.