In order to enhance the rate of crystallisation of polyhydroxybutyrate (PHB), and copolyhydroxybutyrate-co-valerate (CHB/HV), the effect of various seeding agents was investigated. Solid mixtures of PHB containing either saccharin or phthalimide, and a blend of CHB/HV with PHB were prepared. The effect of these materials as nucleating agents was compared with a commercial nucleating agent, boron nitride (BN). In assessing the relative seeding abilities, overall crystallisation rate, spherulite development and nucleation density were measured by DSC and optical microscope. FT-IR microscopy was used to determine the solubility of the nucleating agent in the molten polymer. The conclusions are drawn that saccharin and phthalimide are much less effective as nucleating agents than BN. They are comparatively soluble in molten PHB but are rejected from the crystalline PHB. There was little nucleation by saccharin and phthalimide, and whilst small crystallites of saccharin and phthalimide develop within the boundaries of the spherulite much of the material accumulates in the regions between the spherulites. PHB acted as a nucleating agent for CHB/HV although the activity was not found to be superior to BN as the nucleating agent.