In this paper I examine mixed synchronous and asynchronous text-based conversations that have been carried on in the context of a computer-mediated communication (CMC) system called "Babble", which has been in use by a group of nine-teen people for nearly two years. The primary goal is to explore principled ways of analyzing and characterizing conversational activity in such systems using genre theory. After discussing genre theory, and some of the issues that come to the fore when apply genre theory to CMC, the paper analyzes five conversations. It argues that the conversations constitute separate genres, and develops the concept of participatory structure to capture some of their differences. Next, the paper examines the CMC system as a whole: it argues that the CMC system may be viewed as an ecology of conversational genres, and discusses three properties - global pull, topical pull, and conversational impetus - which may be used to characterize the behavior of the ecology.