Fusion reactivities ⟨&sgr;v⟩ in the mean ion energy range 0.1–200 keV have been calculated for the reactions: D(d,n), T(d,n), D(d, p),3He(d, p) and T(t,2n); and for several different ion energy and angular distributions considered plausible in hot, magnetically confined plasmas. Calculations were carried out to a numerical accuracy of about 5% and the resulting variation of reactivity with mean ion energy is found to be well represented by a simple polynomial function. The polynomial functions are plotted as a function of mean ion energy and their coefficients given. Results of the calculations show that fusion reactivity is very sensitive to the functional form of the speed distribution at mean energies below a few keV, but reactivity is roughly independent of the form of the speed distribution (within a factor of 2) above 10 keV for most of the reactions investigated. Similarly, the importance of the ion angular distribution is large at energies below a few keV but small at high ion energies.