An investigation on the infection by Trichinella worm in pigs was conducted in 17 villages of 4 communes of Bac Yen district, Son La province (where human cases were recorded) in 2009. From a total of 1035 pig serum samples. the average of the ELISA sero-positiveness was 19.9 percent. The infection was found in pigs of all age and of both sex. However, the prevalence was found in the females higher than in the males and increasing as a function of the age. The infection was found in all the 17 villages studied. Given a household as a unit, the average positiveness was 38.9 percent and in the Lang Cheu village where the human cases were recorded, the prevalence was the highest. 53.8 percent. The larvae was found in 50 out of 76 (65.78 percent) of muscle samples taken from the seropositive pigs. The parasite was identified as T. spiralis by studying the larvae in reference laboratories. Fighting measures applied by the DAH consisting of culling sera-positive pigs were found effective as indicated by the no recurrent cases in the following three years.