Five viruses causing colour-breaking of tulip flowers were isolated from tulips and lilies. Tulip-breaking virus (TBV), tulip top-breaking virus (TTBV), tulip band-breaking virus, Rembrandt tulip-breaking virus and lily mottle virus were all characterized as potyviruses by serology and potyvirus-specific PCR. Sequence analysis of amplified DNA fragments spanning a conserved area of the coat protein cistron of potyviruses was performed in order to classify the isolates as distinct viruses or strains. It appears that all tulip-breaking viruses are distinct viruses and TTBV was found to be strain-related to turnip mosaic virus.