We have analyzed the reactivity of a 217 base pair segment of the intrinsically curved Crithidia fasciculata kinetoplast DNA towards eukaryotic DNA topoisomerase I. The substrates were open [linear fragment and nicked circle] and closed minidomains [closed relaxed circle and circles with linking differences of -1 and -2]. We interpreted the results with the aid of a model that was used to predict the structures of the topoisomers. The modelling shows that the delta Lk(-1) form is unusually compact because of the curvature in the DNA. To determine the role of sequence-directed curvature in both the experimental and modeling studies, controls were examined in which the curved Crithidia sequence was replaced by an uncurved sequence obtained from the plasmid pBR322. Reactivity of the Crithidia DNA [as analyzed both by the cleavage and topoisomerization reactions] markedly varied among the DNA forms: (i) the hierarchy of overall reactivity observed is: linear fragment > nicked circular, closed circular [delta Lk(0)], interwound [delta Lk(-2)] > bent interwound [delta Lk(-1)]; (ii) the intensity of several cleavage positions differs among DNA forms. The results show that eukaryotic DNA topoisomerase I is very sensitive to the conformation of the substrates and that its reactivity is modulated by the variation of the compactness of the DNA molecule. The C. fasciculata sequence contains a highly curved segment that determines the conformation of the closed circle in a complex way.