Small-angle neutron scattering profiles from linear polystyrene-polyisoprene block copolymers were measured as a function of temperature. Measurements were made on eight diblock copolymers and two triblock copolymers, with molecular weights ranging from 1.2 x 10(4) to 4.1 x 10(4) and phi ranging from 0.13 to 0.46 (phi is the volume fraction of the minor component). The scattering profiles were consistent with the mean-field theory of Leibler for block copolymers with 0.25 < phi < 0.5. Fluctuation corrections proposed by Fredrickson and Helfand were found to be increasingly important as the order-disorder transition was approached. However, large deviations between experiment and theory were observed in low molecular weight asymmetric block copolymers with phi < 0.25. The amplitude of the concentration fluctuations was found to be an order of magnitude larger than predictions by Leibler, and Fredrickson and Helfand, These findings cannot be explained on the basis of a composition-dependent Flory-Huggins gamma parameter because this anomaly is not evident in a higher molecular weight block copolymer with the same asymmetry (phi).