In an experimental tank (ET), three experimentally naïve lake sturgeons (Acipenser fulvescens) were operantly conditioned to enter one of four target areas where they received an auditory response-feedback stimulus (RFS) in the form of a click sound and – according to the prevailing FR schedule – 10 seconds of darkness. A multiple-baseline-across-subjects design was used. Visual observation and a video-tracking system (VTS) monitored the number of responses emitted on each of the four target areas. The target areas were in the corners of the ET and had to be entered by the subject to constitute a response. The experiment generally consisted of the following phases: (1) no-feedback baseline (NFB) phase; (2) discriminative stimulus baseline (DSB) phase; (3) response-feedback baseline (RFSB) phase; and (4) fixed-ratio phase (FR n). The data indicated a preference for the target area that produced darkness, which suggests that darkness is a reinforcer for this species. This preference was indicated by a greater number of target area responses for the target area whose entry into it produced darkness, and greater activity in and around that area relative to baseline phases. Furthermore, when the ratio of the FR n phase was increased the subject’s rate of responding also increased.