Kinetoplast DNA minicircles from Crithidia fasciculata contain a single major region of bent helix. Restriction fragments containing this bent helix have electrophoretic behavior on polyacrylamide gels which is much more anomalous than that of previously studied bent fragments. Therefore, the C. fasciculata fragments probably have a more extreme curvature. Sequencing part of a cloned minicircle revealed an unusual structure for the bent region. In a sequence of 200 bases, the bent region contains 18 runs of 4-6 As with 16 of these runs in the same strand. In some parts of this sequence the A runs are regularly spaced with a periodicity of about 10 base pairs. This spacing is nearly in phase with the twist of the DNA helix. This same sequence arrangement has been observed in other bent fragments, but the number of A runs is much greater in this C. fasciculata sequence. It is likely that there are small bends associated with each A run which, because of their periodic spacing, add up to produce substantial curvature in this molecule. In addition to having highly anomalous electrophoretic behavior, the fragment has unusual circular dichroism spectra. Its spectrum in the absence of ethanol is that of B DNA, but ethanol in the concentration range of 51-71% (w/w) induces changes to forms which are different from those of any well characterized DNA structure. The C. fasciculata bent helix is neither cleaved by S1 nuclease nor modified by bromoacetaldehyde under conditions in which other unusual DNA structures (such as cruciforms or B-Z junctions) are susceptible to attack by these reagents. Finally, a two-dimensional agarose gel analysis of a family of topoisomers of a plasmid containing the bent helix revealed no supercoil-induced relaxation.