  The nonleptonic weak $|\Delta S|=1$ $\Lambda N$ interaction, responsible for
the dominant, nonmesonic decay of all but the lightest hypernuclei, is studied
in the framework of an effective field theory. The long-range physics is
described through tree-level exchange of the SU(3) Goldstone bosons ($\pi$ and
$K$), while the short-range potential is parametrized in terms of lowest-order
contact terms obtained from the most general non-derivative local four-fermion
interaction. Fitting to available weak hypernuclear decay rates for $^5_\Lambda
{\rm He}$, $^{11}_\Lambda {\rm B}$ and $^{12}_\Lambda {\rm C}$ yields
reasonable values for the low-energy constants.
