Publisher Summary Gene expression studies are very important in the study of marine heterotrophic bacteria, particularly the extremophiles, in terms of understanding the response to pressure and low and high temperature. The capability to detect gene expression by mRNA analysis has enabled the real-time detection of expression of genes involved in bioremediation as well. Gene expression encompasses transcription, translation, post-translational modification, and production of a functional gene product. This chapter describes three related methods for mRNA isolation and compares some results obtained by each. Two RNA extraction methods; Guanidinium isothiocyanate phenol sarcosyl (GIPS) method and Boom methods are discussed. Dotting, probing, quantitating mRNA levels, and application of assay are also described. Extraction of natural populations of phytoplankton from Ft. Desoto, St Petersburg, FL and probed with the Form ID probe indicated that the Boom protocol resulted in hybridization that was at least as great as that with the GIPS reagent.