Constitutive heterochromatin was observed in Lathyrus aphaca, L. cicera, L. odoratus, L. sativus and L. tingitanus. C-bands in L. odoratus and L. tingitanus are centromeric or proximal in location. In L. tingitanus, only the short arm of Chromosome 3 showed a large telomeric band; this was detected by C-banding following Q-banding but not in directly C-banded preparations. In L. odoratus, while most C-bands showed as positive Q-bands with enhanced fluorescence, small telomeric bands showed no differentiation with quinacrine. In L. tingitanus, constitutive heterochromatin showed two types of Q-band: one negative with quenched fluorescence at the centromeric regions of all the chromosomes and the other, positive with enhanced fluorescence at one or both sides of the negative bands. No other species of Lathyrus investigated showed discrete negative Q-bands. In L. aphaca and L. cicera. Q-banding patterns are characterized by large telomeric positively fluorescent bands on most members of each complement. Some chromosomes have proximal and/or intercalary bands. In L. cicera, some C-bands showed no differentiation with quinacrine staining. In L. aphaca, a large positive Q-band normally at the end of the short arm of Chromosome 6 is sometimes replaced by two small intercalary bands. Prominent telomeric positive Q-bands were also detected in L. sativus together with small proximal bands and intercalary bands. Whole short arms of Chromosomes 3 and 4 and the long arm of Chromosome 6 in L. sativus showed slightly quenched quinacrine fluorescence. The evolutionary implications of these observations are discussed.