Summary: The distribution of kamacite, taenite, and plessite suggests that kamacite nucleated at the sulphides, which appear to have been plentifully dispersed in the parent taenite. Such a process could take place without the undercooling that is usually assumed in normal octahedrites. The plessite fields show internal boundaries that are probably relics of grain boundaries in the parent taenitesulphide aggregate. Microprobe investigations have been made of nickel distributions in and between kamacite, taenite, plessite, and schreibersite, and particular attention has been paid to the nickel distribution at the internal grain boundaries in the plessite. Evidence is presented for the diffusion of nickel along these boundaries and for their migration during the later stages of formation of the microstructure.