Cryptosporidium is a common protozoan pathogen with a worldwide distribution causing self-limited diarrhea in immunocompetent individuals and chronic and life-threatining diarrhea in immunocompromised patients. The aim of this study was to determine the prevalence of cryptosporidium in families in a slum district in Mersin by using simple random sampling method. The study was carried out in 76 families (466 persons) consisting of subjects without diarrhea and healthy. Of the people 105 (22.5%) were not voluntary to give fecal samples. Totally 361 fecal samples were collected from families. All the fecal smears were prepared by formalin-ether sedimentation and examined by light microscopy after modified Kinyoun's acid-fast methods. Cryptosporidium oocysts were observed in 11 of 361 persons (3.1%) and in 9 of 76 families (11.8%) in at least one person. It was seen that the average monthly income of families was lower than the normal standard and all of the families obtained drinking water from city regional utilities. There was no statistically significant difference between cryptosporidium positivity and number of members and children in household, sex and age (p>0.05), but there was statistically significant differences between families with 4-6 persons and families with 1-3 persons and over 7 persons (p<0.05). The prevalence of cryptosporidium among families with outdoor toilet, lacking washbasin was found to be higher than families having indoor toilets and washbasin. However, this difference was not significant (p>0.05). This study showed that cryptosporidium positivity was high in Mersin slum district and that the transmission of parasite depended on economical status, education level, hygiene and sanitation facilities.