We study an economy with traders whose payoffs are quasilinear and whose private signals are informative about an unobserved state parameter. The limit economy has infinitely many traders partitioned into a finite set of symmetry classes called types. Market mechanisms in a class that includes auctions yield the same outcome as the Walrasian rational expectations equilibrium if and only if the efficient allocation has a monotonicity property. Examples illustrate cases where they differ. Monotonicity restricts the heterogeneity among traders’ types.