The length of cooperative units along the thin filament of rabbit psoas single muscle fibres was determined by reducing filament length by treatment with the thin filament severing protein, gelsolin, in the presence of Ca2+ and 2,3-butanedione 2-monoxime (BDM). The average time for 50% reduction in isometric tension was 6.7 min at 22°C. The pCa–tension relationship was measured at 22°C, pH 7.00 and ionic strength 200 mM, and the data were fitted to the Hill equation to determine the half-saturation point (K) and the cooperativity (n). Our results demonstrate that the cooperativity does not change much when the remaining isometric tension was in the range 20–100%. The cooperativity quickly diminished when the remaining tension was reduced to less than 20%. Our results further demonstrate that the change in the pK value was minimal and averaged 0.075 (less Ca2+ sensitive) as the thin filament length was reduced. We infer from the first observation that the thin filament cooperativity extends up to 0.2 μm, which includes the maximum of about four basic cooperative units consisting of seven actin molecules, one tropomyosin dimer and one troponin complex. We infer from the second observation that the Ca2+ sensitivity is slightly reduced by removal of the cooperative interaction between neighbouring cooperative units.