Fermentation of yoghurt is the most important part of the production.For fermentation process ,streptecoccus thermophilus and lactobacillus bulgaricus are added at 1:1 ratio as starter culture.There are also other bacterias in the product such as mold and yeast ,lactic acid bacteria etc. Detection of these microorganisms has an important role for safety of products. In this experiment we have used two kinds of yoghurt sample prepared in aceptic conditions and in open environment.We have examined the differences in the number of microorganisms in these samples.We have used different agars fort hat purpose.That is because,total lactic acid bacteria can be determined with MRS agar ,s.thermophilus and lactobacillus bulgaricus can be differentiated by using lee’s agar. Because , s.thermophilus ferment sucrose and produce yellow color but l.bulgaricus can’t so that we can differentiate them from each other by using lee’s agar.In M-17 agar, reproduction of s.thermophilus bacterias is observed.In PCA agar, at 30 °C microorganisms other than lactic acid bacterias can be detected.Also during sample preparation, we diluted our sample with a different way because that our sample was solid so we took 25 gr and dissolved it in 225 ml of sterile water to obtain a well mix.In the results,the number of microorganisms in open yoghurt are greater than the number in pasteurized yoghurt. That is because,in pasteurized yoghurt,the yoghurt product is pasteurized during the production so the number of microorganisms is reduced because of heat threatment and aceptic packaging.But in open yoghurt,it is produced in open environment so the product is exposed to air and other factors such as human interferences and non aceptic packaging.However,we have also observed that the results are a bit strange and unacceptable.Because when yoghurt samples are diluted and inoculated ,the obtained number of microorganisms are greater in so diluted samples such as 10-5 such as 10-2 because of wrong calculations or careless experiment work etc. or 10-6 than less diluted samples or 10-3 .This is not an expected case.This mistakes may be

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