Microscopical studies have shown that cyanobacteria, particularly, Oscillatoria spp., Lyngbya-Phormidium spp. and Microcoleus spp. and purple sulphur bacteria were able to grow in intimate contact with Maya and Casablanca crude oil but there is little experimental evidence to indicate that they are actively involved in hydrocarbon degradation. The majority of the heterotrophic hydrocarbon strains isolated were Gram negative although a number of Gram-positive isolates have also been isolated and identified as Rhodococcus spp. and Corynebacterium spp. The Gram negative isolates were initially characterised using conventional methods (API bioMérieux and Biolog GN2) and identified as Pseudomonas spp., Acinetobacter spp. and Halomonas spp. Molecular characterisation of these isolates by the Dutch group has shown that several of the isolates we had identified as Pseudomonas spp. are Marinobacter spp. It is clearly evident from this study that Marinobacter spp. are more widely geographically distributed in the microbial mats than had been previously known. Physiological studies on selected isolates show that they are remarkably salt tolerant and will grow under both hypo- and hypersaline conditions. All the isolates are mesophiles and grow optimally in the temperature range 28-35 oC. Screening of the isolates has shown that straight chain hydrocarbons were the most widely utilised substrates and only a limited number were able to degrade mono-aromatic hydrocarbons. The Rhodococcus isolates degrade both individual hydrocarbons and defined hydrocarbon mixtures very rapidly (up to 70 µmol.d-1) and produce large quantities of cell bio-surfactants, which enhance oil degradation. Characterisation of the properties of the bio-surfactants produced by the Rhodococcus has been carried out by our French partners in Pau.