The ultrastructural changes in type 1 (red) and type 2 (white) myofibres of the longissimus muscle were studied during in situ postmortem ageing in a group of young Hereford bulls, 14-16 months old. Type 1 myofibres were defined as those having wide, dense and distinct Z-discs and type 2 myofibres were those with narrower and less distinct Z-discs. Type 1 myofibres also had numerous intermyofibrillar mitochondria and type 2 myofibres had few intermyofibrillar mitochondria. Three time periods were studied-1 h, 48 h and 216 h postmortem. Type 1 myofibre Z-discs were essentially unaltered at all three postmortem time periods studied. Type 2 myofibres showed limited degradation (loss) of Z-disc material at 1 h postmortem, but degradation had increased at 48 h. However, little additional Z-disc degradation occurred between 48 h and 216h. Both type 1 and type 2 myofibres underwent myofibrillar fragmentation at the level of the Z-disc and I-band which was independent of Z-disc degradation. The most tender longissimus muscles tended to have more fragmentation at the Z-disc-I-band junction than the toughest muscle samples.