Although the kinetics of biological oxidation of ferrous to ferric iron - the key step in any bioleaching process - have been studied for a variety of organisms, the focus has always been on conditions typical of tank-bioleaching. In heaps, parameters such as pH and temperature vary widely across the heap and are usually far from optimal. Total iron concentrations are usually much lower than in tanks (except in gold heaps), and the presence of dissolved gangue mineral is usually much more significant. Furthermore, the micro-organisms tend to accumulate on the ore surface and propagate only slowly. In the present paper we report on continuous culture tests using the bacterium Leptospirillum ferriphilum - predominant in most commercial bioleach operations - at temperatures 20 - 42°C, pH 0.8 - 2.0, [FeT] = 2-12 g/L and solutions of MgSO4 and Al2(SO4)3 of ionic strength from 0.2 to 1.3 M. The results clearly indicate that under typical heap solution conditions ferrous iron oxidation kinetics of planktonic micro-organisms are retarded to at least some degree. Ongoing tests are focussed on the kinetics of attached bacteria, with preliminary results indicating that these are noticeably less efficient at ferrous iron oxidation when compared to planktonic cells. This work bears significant implications for the design of heap bioleach operations to ensure that a much more optimal environment for microbial growth and oxidation is created within heaps than is currently practised.