Condition index is a good indicator of mussel’s state of health and the condition of the ecosystem they live in. The purpose of this study is to obtain data on the quality of farms mussels and mutual comparison and ranking of farms by determining of the condition index. Condition index represents the proportion of meat in the total mass of shellfish or shells. Mediterranean mussel Mytilus galloprovincialis is known as a biological indicator organism. As a sessile organism and filter feeder it indicates the presence or absence of contaminants in the sea, and it can also be used for monitoring at periodic changes in pollution levels in certain areas. Mussel samples were taken at five sites in Istria: Lim channel, Raša bay, Vabriga, Budava Bay and Pomer bay. In this study the condition index was calculated from the wet weight of tissue and volume of shells (IK-1), shellfish mass (IK-2), mass of shells (IK-3) and the dimension of the shells (IK 4 and IK 5). All condition indexes confirmed that mussels from all sites are of good quality with the content of the meat of about 15%. Comparing of the sampled mussels in the winter period, we rank them from the highest condition index value to the lowest: Pomer bay > Vabriga > Raša bay > Budava > Lim channel. High values of condition index (with the 25% content of the meat in shells) for the winter period were in Pomer suggesting the best environmental conditions in the farm. Comparison of the mariculture sites during the summer we ranked sites: Budava > Lim channel > Pomer bay > Raša bay > Vabriga. Mussels from Budava in the summer period have the highest condition index values and the meat content of 21%. The best mussels for consumption in the Istria for the winter periods are the mussels from Pomer bay, and for the summer periods are the mussels from Budava.