A mechanism is suggested for twist-banding in spherulites of poly(3-hydroxybutyrate) (PHB), which involves elastic bending of lamellar ribbon crystals caused by chainfolding in different directions on opposite faces of lamellae. This folding behaviour is considered to be a consequence of the stereoregularity of bacterially produced PHB, whose molecules adopt a helical chain form. The model is used to predict values for the band spacing P in terms of crystal elastic moduli, lamellar surface energy density and lamellar thickness. Measured values of P are somewhat larger than predicted, and too small a temperature dependence is predicted at low supercoolings. Reasons for the discrepancies are suggested and discussed. This proposed mechanism for helicoidal twisting could be appropriate for many polymers, in particular for chiral polymers, and in cases where fold staggering does not play a role.