BACKGROUND: Cryptosporidium parvum is a protozoan parasite which belongs to apicomplexa phylum. The parasite infects both wild and domesticated animals and human beings as well. OBJECTIVES: The purpose of the present study was to detect oocyst shedding and diarrhea pattern in experimental cryptosporidiosis and their correlation with weight loss in neonatal calves. METHODS: Twelve Holstein calves of both sexes were obtained at birth from dairy farm and randomly divided into two groups of 6 calves. Six calves were orally infected with 107 C.Parvum oocysts at the 12h post parturition. The control group was not infected. Clinical signs were examined and fecal samples were collected by the rectal examination twice a day. All calves were weighed from day 0 to day 30 with 3 days intervals to determine effects of cryptosporidiosis on weight gain. RESULTS: All infected calves were noticeably depressed and had a decreased appetite from 3 days post inoculation (DPI) while they received colostrum. Subsequently, watery diarrhea with clumps of mucus and yellow or pale changes of feces color were observed. The infected calves have had diarrhea for 5-8 days that remarkably had got dehydrated. The most severity of diarrhea was 4-6 DPI. Oocyst excretion started 4 DPI, peaked at 6 DPI (60.48×106±9.03oocysts/g feces) and continued until 11 DPI. Control calves had no diarrhea and other clinical signs during the whole period of the trial. The mean weight gain of control group was significantly higher than inoculated group during experiment (p<0.001). The Weight of the infected calves was retarded until 9 days old and then risen subsequently. CONCLUSIONS: Present study showed the role of C.Parvum as the primary cause of diarrhea and weight loss among neonatal calves.