Spatial and temporal variability in bacterial biomass, activity, and production were examined within the hyporheic zone beneath a riffle-pool sequence of a northern Michigan river. Biomass and production were low at all sites and depths during November and February but increased by an order of magnitude in May. Activity (the proportion of respiring cells to total cells) was highest (18%) during November and generally < 1% by February and May at all sites. The site farthest upstream beneath the riffle was the most active during November and most productive during May. Carbon turnover times were slowest in February and fastest in May. Activity and production did not correlate with amounts of sediment organic matter or interstitial dissolved organic site. Seasonal differences might be attributed to temperature variations. A hypothesis for the observed spatial differences within a season might be attributed to local conditions in the subsurface environment resulting from surface water advection (downwelling zones) or groundwater (upwelling zones).