To speak of substrate usually brings to mind the idea of a surface into or upon which organisms are modified to burrow, hide, cling or feed, and not usually its equally important effects upon water currents and velocity, upon turbulence production for mixing and aeration, as a blanket affecting respiration of benthos or as a stimulant of respiratory rate of the associated organisms. To consider the substrate in these latter ways involves more than an immediately obvious effect upon the organisms. The paper uses the oxygen parameter as a point about which to center the discussion of substrate, current and oxygen. The occurrence of oxygen in microhabitats and how it is affected by water current and substrate is related. The effect of the oxygen concentration, the substrate and the current velocity upon organism respiration is then discussed. Because gross measurements of the environment are of little value to the understanding of the micro-environment, a plea is made for determination of the parameters within the microhabitats of the given organisms. The necessity of such measurements is demonstrated by the few works thus far attempted with the limited techniques available.