The objective of the study was to investigate pathogenesis of paratuberculosis in sheep experimentally infected with a caprine isolate of M.a. paratuberculosis (MAP) and also the transmission potential of the infected sheep to the in-contact sheep over a period of 390 days post-infection (DPI). Cross-bred lambs, 8-12 weeks old, negative to faecal culture/PCR and ELISA to MAPinfection were divided into infected (15) and in-contact (8) groups. Lambs were orally infected with 4x10⁹ MAP ten times within a period of one month and the in-contact animals similarly received sterile phosphate buffered saline. A new classification system based on the results of a set of diagnostic tests (faecal smear examinatioin, culture and PCR, tissue PCR, and ELISA, immunoperoxidase and histopathology) were used to categorise infected and in-contact animals. Amongst 15 infected sheep, 2 (13.3%) each were found to be in the negative and the suspected, 5 (33.3%) in the mild and 3 (20%) each were in the moderate and the severe infection categories. Four (50%) sheep of the in-contact group had mild infection. Gross lesions consisting of mild to severe thickening of intestinal mucosa, enlarged mesenteric lymph nodes and gelatinisation of fat could be observed at 150 DPI. Up to 90 DPI, histological lesions were not detected, which was a major point of difference from earlier experiments. The mild infection lesions were characterised by the presence of focal granulomas, mostly in the Peyer's patch area of the ileum and ileocaecal valve (ICV) and occasionally multinucleated giant cells without demonstration of acid-fast bacilli. The moderate infection lesions consisted of multiple focal granulomas containing few to numerous AFB from proximal jejunum to the ICV, besides diffused infiltration of lymphoid cells and macrophages. The lesions in Peyer's patch areas were more severe. The severe lesions varying from large multifocal epithelioid granulomas to formation of epithelioid cell sheets containing abundant AFB were observed in 3 sheep and were similar to the classical lesions of paratuberculosis. It was concluded that caprine strain could produce characteristic lesions in sheep after 150 days of infection and in-contact sheep could develop only mild infection. Pathogenicity of caprine isolate of MAP to sheep and its implication in relation to India where sheep and goat husbandry goes side by side has been discussed.