Fission rates have been experimentally determined in the NISUS standard fast assembly, installed on the University of London Reactor CONSORT, with a Makrofol solid-state track recorder (SSTR). The results are compared with fission chamber measurements. The fission tracks were counted by the Quantimet 720. The optical efficiency of the Makrofol SSTR was found to be (94.6 ± 0.7)% for thin deposits. The measurements of 238U/235U fission ratio with SSTR showed a discrepancy of ∼3%o compared with those of the fission chamber.