This study included investigating fungal contamination in samples of malt beverage Available in Mosul With a focus on detecting cyclopiazonic acid and fungal alkaloids and testing the effectiveness of thyme oil in reducing this pollution The fungi were isolated and identified microscopically and morphologically. The use of Ehrlich's reagent as a colorimetric method for detecting the production of toxins and alkalis In both the malt beverage samples and the fungi isolated from it An experiment was also conducted to evaluate the inhibitory effect of thyme oil. By comparing the growth diameters of fungal isolates before and after treatment with oil at different concentrations And documenting that photographically The contamination level in the samples reached (77.5℅) The Aspergillus was the most dominant in (42.8℅) . The detector demonstrated its efficiency, as the results showed a positive response to the detector. The color red appeared in the sample R 12 and the color purple appeared in the sample R19 and the color yellow appeared in the sample R*.The fungal isolate Aspergillus fumigatus also showed a red color reaction ,Penicillium expansum give a purple color. And Cladosporium herbarum reacted yellow color. This confirms its contamination with toxins cyclopiazonic acid and other alkaloids Thyme oil demonstrated very high inhibitory efficiency, achieving a maximum inhibition rate of 100% against fungal growth at the specified concentration 0.5℅ This confirms the oil's effectiveness in inhibiting fungal growth. This opens up prospects for its use as a natural preservative in local food industries in the city of Mosul.