An exhaust-gas sampler is adapted to couple into an exhaust conduit and to sample a constituent of an engine exhaust flowing therein. The sampler comprises an envelope having upstream and downstream surfaces joined by a curved side surface. The upstream and downstream surfaces are each tangent to a plane substantially normal to a central axis of the exhaust conduit. A series of inlets are formed in the upstream surface, and an outlet is formed in the curved side surface. A sensor responsive to a level of the constituent in the exhaust is coupled in the envelope, at the peripheral region of the conduit.