Supernumerary (B) chromosomes have been studied in a Spanish population of Scilla autumnalis L. (Liliaceae). Out of the 140 individuals analysed, seven had 2n=14+1B, one 2n=14+2B, one 2n=14+3B and one 2n=14+9B. An analysis of esterase isozyme patterns shows that all 130 individuals with a standard karyotype (2n=14) have two esterase loci, Est-2 and Est-3, whereas all 10 individuals with Bs have three, Est-1, Est-2 and Est-3, irrespective of the precise number of Bs present. The role that the Bs may have played in the appearance of this new locus (Est-1) is discussed in relation to their possible origin.