A new solid medium, termed solid 2:2 medium, was developed for application to the genetic manipulation of Thiobacillus ferrooxidans. The medium contains a mixture of both ferrous iron and thiosulphate as energy sources for the growth of T. ferrooxidans at pH 4.6-4.8. Kanamycin and streptomycin could be used in the medium to select recombinants. Thousands of interspersing colonies can develop on each plate without any iron-oxidizing zones. T. ferrooxidans colonies on solid 2: 2 medium have distinct morphologies, and several types of colonies were described. A new type of colony morphology variant with high mutation and reversion rates was also observed. © 1994, Applied Microbiology, Molecular and Cellular Biosciences Research Foundation. All rights reserved.