Supplementation of the divalent cations calcium and magnesium to submerged cultures of Streptomyces hygroscopicus var. geldanus greatly influenced morphological development and secondary metabolite synthesis. The disparate response could be explained in terms of the differential effects of Ca2+ and Mg2+ ions on cell surface hydrophobicity. Cultures supplemented with calcium ions were found to be hydrophobic, which resulted in cell concentration-dependent aggregation. In contrast, those grown in a magnesium-rich medium were found to be hydrophilic with the organism growing as freely dispersed filaments that synthesised geldanamycin at an optimal rate in comparison to hydrophobic pellets.