The tensile properties of single muscle fibres isolated from porcine longissimus and iliocostalis muscles conditioned for various lengths of time and cooked at different temperatures for 1 h were investigated. The stress-strain curves of fibres extended to fracture were r-shaped with yield points of 2-5% resting length in raw fibres and 10-20% in cooked fibres. The yield point was independent of cooking temperature, fibre diameter and mechanical skinning of the fibres but decreased with ageing. The stress and strain at which the muscle fibres fractured was found to depend on cooking temperature, fibre diameter and conditioning time. Raw muscle fibres isolated from 24 h aged longissimus and iliocostalis muscles required 216 & 13 kN m-' and 272 f 6 kN m-2, respectively, to fracture. On cooking the muscles for 1 h at 80Β°C the fracture forces increased to 342 f 19 kN m-' in fibres isolated from the longissimus and 443 f 36 kg cm-2 in fibres isolated from the iliocostalis. Ageing the muscles for 72 h halved the forces required to break raw muscle fibres isolated from them. However, further ageing of the muscles had no noticeable effects on the force required to fracture fibres isolated from the longissimus dorsi but caused a further reduction in the stresses needed to break fibres isolated from the iliocostalis muscle. Muscle fibres isolated from the iliocostalis fractured at higher strains than those isolated from longissimus muscle. However, in both muscles the strain at which the fibres fractured increased with cooking temperature and was lowest in fibres isolated from raw muscles and highest in fibres isolated from muscles cooked at 80Β°C for 1 h. Ageing the muscles led to a marked decline in the strain at which the fibres fractured and these effects were greater in fibres isolated from the longissimus than in fibres isolated from the iliocostalis muscle. In 24 h and 72 h conditioned muscles, fibres having diameters < 60 pm required significantly greater forces to fracture than those with larger diameters. However, ageing the muscles for 264 h removed this diameter-dependent difference in fracture force. Fibre diameter had no effect on the strain at which the muscle fibres fractured at all experimental conditions.