Three experiments are reported, dealing with a situation in which subjects carried on a dialogue, via a computer terminal, with what they believed to be either a computer system or another person. Analysis of the ensuing protocols concentrated on the use of anaphor and on lexical choice. Systematic stylistic variations result from placing subjects in a situation where they believe their interlocutor to be a computer system. Subjects in this condition sustain a dialogue with a restricted window of focussed content and, as a result, compared to person-to-person dialogues, utterances are short; lexical choice is kept to a negotiated minimum; and the use of pronominal anaphor is minimised, even when referring to the discourse topic. These dialogue characteristics persist over lengthy periods of interaction. Lack of evidence to support presuppositions concerning the capabilities of a defined computer source does not lead to a change in style. Similarly, attempts to manipulate features of computer output by producing more friendly surface forms did not influence subjects' behaviour. However, within the limits of the measures taken, subjects learned as much and performed as well, or better, in interactions defined as being with a computer.