A rapid and efficient microspore culture protocol was applied to produce homozygous progeny of crosses between low erucic canola and high erucic resynthesized rapeseed (Brassica napus L.). Microspores of Canadian cultivars ÔExcelÕ and ÔProfitÕ as well as three F 1 hybrids with the resynthetic line ÔRS239Õ were treated with colchicine immediately after isolation. Flow cytometry was applied for early identification of doubled haploid (DH) regenerants. The diploidization rate was subsequently verified by scoring flower morphology. In vitro colchicine treatment had a positive effect on induced diploidization, and was associated with the frequency of preliminary spontaneous diploidization which was, however, determined by the genotype. In addition, the effects of colchicine treatment on embryoid formation and regeneration have been evaluated. The method presented is feasible for commercial large-scale production of DHs in rapeseed as the genotype-specific diploidization can be efficiently balanced by in vitro colchicine treatment. In addition, the use of flow cytometry immediately after in vitro culture allows efficient selection for DHs, thus saving labour and cost and in the laboratory and subsequent greenhouse phase.Key words: Brassica napus -doubled haploids -flow cytometry -induced diploids -ploidy level -resynthetic rapeseed Microspore culture of rapeseed is an established tool for the production of homozygous lines in breeding programmes. For this purpose, fertile doubled haploid (DH) plants at a high and genotype-independent frequency are required. Usually, 70-90% of the microspore-derived plants are haploid (Lichter 1985, Chen andBeversdorf 1992). Mo¨llers et al. (1994) increased the diploidization rate to 80-90% after a 24-h treatment of microspores with 50 mg/l colchicine in the induction medium immediately after isolation. Correspondingly, Zamani et al. (2000) demonstrated the feasibility of an in vitro colchicine treatment in wheat anther culture.The efficiency of microspore culture in plant breeding is restricted by the occurrence of haploid plantlets which require space in the growth chamber and greenhouse and time for colchicine treatment. The determination of ploidy level in an early phase can reduce these efforts to a minimum (Wang et al. 1999). However, conventional methods for early determination of ploidy level, such as counting the mitotic chromosomes or measuring pollen size, have limitations on efficiency and reliability. Flow cytometry enables the measurement of ploidy level in an early developmental stage of plantlets emerging from microspore culture. Therefore, an in vitro colchicine treatment coupled with an efficient determination of ploidy level by flow cytometry enhances the use of microspore culture in rapeseed breeding programmes.Plant material and microspore culture: The present experiment comprised three low erucic (<0.5% erucic acid) Canadian cultivars, i.e. ÔExcelÕ, ÔProfitÕ and ÔTristarÕ, the resynthesized rapeseed line ÔRS239Õ (Lu¨hs and Friedt 1994) and three crosses (F 1 ) (cf...