Coal- and lignite-fired power plants produce significant amounts of fly ash, which in many cases contains high concentrations of naturally occurring radionuclides such as 238U. This is of importance from the radio-environmental point of view since a small portion of the fly ash escapes from the power plant filtering system and disperses in the surrounding atmosphere. In addition, coal- and lignite-fired plants related activities such as fuel mining and ash deposition contribute to the suspended matter in the air with particles containing naturally occurring radionuclides. In this study, rainwater samples were collected in the Megalopolis lignite field basin of the Arcadia prefecture in Greece, where lignite-fired power plants are in operation and related activities are fully developed. The samples were preconcentrated and analyzed with alpha spectrometry and kinetic phosphorescence analysis in order to determine the levels of uranium isotope concentrations in precipitation in the Megalopolis basin. A control rainwater sample was also collected in the Athens area to account for reference purposes. Concentrations determined via the two techniques were found to be in statistical agreement. The concentration of 238U in the samples collected at the Megalopolis basin varies from 2.2 ± 0.6 to 90 ± 14 mBq/L, and is 5-40 times higher than the concentration of 238U in the sample collected in Athens. This could be attributed to the operation of the lignite-fired power plants, the fossil fuel mines, and the ash depositories.