The fatiguability of the quadriceps muscle was investigated in 10 male subjects (25-40 yrs), with inter-individual differences in fibre composition of their vastus lateralis muscles (range 25-65% fast twitch, FT, muscle fibres). Fatiguability was assessed as the decline in maximal force (in % of initial values) with 50 repeated isokinetic knee-extensions at fast angular velocity (3.14 rad/s). Each contraction lasted 0.5 s and the rest periods were about 0.7 s. Every subject was tested on two occasions and the standard deviation for a single determination of fatiguability was 1.4%. The decline in force after 50 contractions was on the average about 45%. The individual values varied, however, and a linear correlation was present between fatiguability and % FT fibres (r = 0.86, p less than 0.01). It was concluded that development of fatigue in human skeletal muscle performing repeated fast dynamic contractions with maximal effort was most marked in muscles with a high proportion FT fibres. This finding was in conformity with earlier results from animal skeletal muscle preparations.