There is unexamined potential for developing and testing rival causal explanations in the type of data that participant observation is best suited to create: descriptions of in situ social interaction crafted from the participants' perspectives. By intensively examining a single ethnography, we can see how multiple predictions can be derived from and tested with field notes, how numerous strategies are available for demonstrating the patterns of nonoccurrence which causal propositions imply, how qualitative data can be analyzed to negate researcher behavior as an alternative causal explanation, and how folk counterfactuals can add to the evidentiary strength of an ethnographic study. Explicating the potential of field notes for causal explanation may be of interest to methodologists who seek a common logic for guiding and evaluating quantitative and qualitative research, to ethnographic fieldworkers who aim at connecting micro- to macro-social processes, to researchers who use an analogous logic of explanation when working with other forms of qualitative data, and to comparative-analytic sociologists who wish to form concepts and develop theory in conformity with an understanding that social life consists of social interaction processes that may be captured most directly by ethnographic fieldwork.