Lactococcus garvieae is the etiological agent of lactococcosis, an emerging disease which affects several fish species and causes important economic losses both in marine and freshwater aquaculture. Lactococcosis usually happens when water temperature increases over 15°C during the year. Normally, it causes a hyperacute and haemorrhagic septicemia in fish. This paper presents a procedure for producing experimental vaccine for rainbow trout (Oncorhynchus mykiss) lactococcosis including aspects such as pathogen characterization, pathogenicity, mass cultivation, safety, potency and field trial tests for immersion use. In the potency test, after challenging the vaccinated fish with live pathogenic bacteria (1×107 bacteria per milliliter of immersing solution) and observing for 72 hours thereafter, 10% of fish died while the control group showed 60% mortality within the observation time. In the field trial from vaccination time onward till marketing of the fish, those mortalities that occurred in groups of vaccinated and non-vaccinated fish were recorded. Total death occurred in the vaccinated group was 11%, while in non vaccinated group this number was approaching 23%. This observation indicates a 50% reduction in mortality in the vaccinated group. This is the first report on experimental vaccine against lactococcosis in fish that is produced and tested in Iran.