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Control of Microbial Ecosystems. Even the most advanced of today's models of biological wastewater treatment systems fall somewhat short of representing the ecology of the various microbial populations present in, say, an activated sludge process. They simulate the interactions between several substrates and several "consortia" of bacterial species, but not the interactions among the bacteria and their higher-order predators. In the long run it is our view that control of such processes, for the practical purpose of more reliable and cost-effective treatment of wastewater, will increasingly draw upon understanding of an ecological, as opposed to merely biochemical, nature. For we know from earlier research that the practical operational problem of a bulking sludge can be successfully addressed with a relatively simple ecological model of the competitive growth of floc-forming and filamentous species of heterotrophic bacteria. What, then, can Control Theory do for this ecological system? It may be, for the time being, that it should do what it normally does: allow us to understand how to cultivate the right organisms in the right relative amounts in the right place at the right time. The distinctive point here is that the humble activated sludge process of wastewater treatment is one of the very few ecological systems in which the exercise of control by external manipulations is both feasible and desirable. It is a laboratory wherein to experiment, learn, and generalize therefrom to larger, grander insights. As long ago as the mid-1970s Thomann — pioneer of environmental systems analysis — introduced an independent variable of trophic length, in his case for the purpose of reducing a burgeoning set of ordinary differential equations to a more manageable partial differential equation set. The distribution of toxic contaminants over space-time-trophic length were to be characterized. For our program, trophic length may have significance in other ways, in permitting distributed-parameter control theory to be employed in identifying how "internal control" in an ecological system is distributed over the continuum of its constituent species, from the smallest to the largest. The microbial ecosystem of the activated sludge process may have too restricted a span of trophic lengths for this notion to be explored. It might better be addressed through the foodweb model prepared for studying the ecological integrity of Lake Lanier, spanning trophic length from bacteria to striped bass fish — from the lowest prey to the topmost predator. This, however, is to extrapolate well ahead of current capabilities; yet we wish to retain such ambition within our sights.