The current study used a stationary acoustic telemetry array to monitor the depth selection of adult smallmouth bass Micropterus dolomieu in a large, oligotrophic lake in Ontario, Canada. At an annual scale, smallmouth bass demonstrated regular, seasonal changes in inhabited depths: fish occupied shallow water during the summer (2–5 m depth) and descended to deeper water (12–15 m depth) during winter under ice. Smallmouth bass remained above the thermocline in the summer, seasonal depth patterns did not vary across fish size and movements to and from seasonal depths were closely linked to the development and degradation of the thermocline. At finer time scales, smallmouth bass exhibited diel vertical migrations in summer, with fish moving to <2 m at night, and then descending to 3–5 m during the day. This pattern remained constant during the summer period examined, and varied with size such that larger fish remained deeper than smaller fish. During winter, depth did not vary across the 3-month monitoring period (c. 14 m), but small (<2 m) changes in depth were observed periodically, suggesting limited movements were occurring. Results are further discussed in the context of climate change and reproductive success for this species.