The mechanism by which amplifying salt fingers in an unbounded uniform T/S gradient are equilibrated is determined, starting with a time-dependent asymptotic field equation for (Rτ)− 1− 1= ε→ 0, where R> 1 is the T/S density ratio and τ= KS/KT< 1 is the molecular diffusivity ratio. A mode truncation of that equation yields an ODE which shows that the fastest growing finger mode transfers energy to two ‘slave’modes with relatively small vertical scale; the finger mode thereby attains a statistically steady amplitude. The results for τ= 1/3 are compared with spectral solutions of the non-truncated equations in two and three dimensions; the predicted fluxes are testable in sugar (S), salt (T) laboratory experiments.