  In this paper we consider the problem of finding the {\em densest} subset
subject to {\em co-matroid constraints}. We are given a {\em monotone
supermodular} set function $f$ defined over a universe $U$, and the density of
a subset $S$ is defined to be $f(S)/\crd{S}$. This generalizes the concept of
graph density. Co-matroid constraints are the following: given matroid $\calM$
a set $S$ is feasible, iff the complement of $S$ is {\em independent} in the
matroid. Under such constraints, the problem becomes $\np$-hard. The specific
case of graph density has been considered in literature under specific
co-matroid constraints, for example, the cardinality matroid and the partition
matroid. We show a 2-approximation for finding the densest subset subject to
co-matroid constraints. Thus, for instance, we improve the approximation
guarantees for the result for partition matroids in the literature.
