Piscirickettsiosis is the most important disease affecting farmed salmonids in Chile since 1989. It is produced by a rickettsial agent named Piscirickettsia salmonis which has been described in all species of salmonids reared in seawater. The effect of water temperature (ºC) and fish loading density (k/m3) on piscirickettsiosis was evaluated. Rainbow trout (Oncorhynchus mykiss) (n=300) were inoculated by intraperitoneal injection with P. salmonis (0.2 ml in 105.8 TCID50/ml (LF-89 strain) and divided into three groups (n=100 each one) according to water temperature (8, 14 and 18 ºC). Subsequently, each group was divided in two according to loading densities (5 k/m3 and 20 k/m3); 24% cumulative mortality was obtained in the group with highest density (20 k/m3) and 14ºC. This was significantly higher than the other groups (p - 0.001). These results suggest that both, temperature and loading density, and their interactions, are significant epidemiological factors for the disease