The rate of magnetite deposition on a heated test section has been investigated using radiotracing methods as a function of flow rate in the absence and presence of a growing biofilm of Pseudomonas fluorescens. The flow rate was adjusted to span Reynolds numbers from 2200 to 9600. For all flow rates there was an increase in the rate of magnetite deposition in the presence of the growing biofilm. In addition, the rate of deposition was an order of magnitude greater for a Reynolds number of 6400 than was observed at lower and higher flow rates with Reynolds numbers of 2200 and 9600, respectively. The results are discussed in relation to the shear stress on the biofilm and to the rate of transport of nutrients.