In this work, we used the random amplified polymorphic DNA (RAPD) technique to evaluate the genetic diversity in Lactococcus garvieae, an important pathogen for fish. Fifty-seven strains with different hosts and geographical origins, including Japan and several countries of the Mediterranean area such as Spain, Portugal, France, Italy, England, and Turkey, were analyzed. Two primers, oligonucleotides 5 and 6 (Pharmacia Biotech) were utilized; primer 5 was the most discriminative, since allowed us to differentiate 10 RAPD -types related to the origin of the strains. Regardless of the oligonucleotide primer employed, the 57 isolates of L. garvieae studied were separated into three genetic groups, composed of the Spanish, Portuguese, English, and Turkish strains (group A), the Italian and French strains (group B), and the Japanese strains (group C). The similarity of isolates within each group, estimated on the basis of the Dice coefficient, ranged from 75 to 100%. Our findings also indicate that RAPD profiling constitutes a useful tool for epidemiological studies of this fish pathogen.Lactococcus garvieae, one of the major gram-positive cocci pathogenic for fish, is considered a serious problem in cultured marine and freshwater fish species such as yellowtail (Seriola quinqueradiata) in Japan and rainbow trout (Onchorynchus mykiss) in Europe and Australia (4,8,(10)(11)(12)(19)(20)(21). In Spain, this disease has appeared in rainbow trout farms since 1991 (9, 28) and at present is considered one of the most important risk factors in the trout industry during the summer months. Besides fish, L. garvieae has been isolated from cows and buffalo (6, 31); it has also been recovered from humans sources (13,14). In addition, it has been isolated recently from diseased freshwater prawns (Macrobrachium rosembergii) in Taiwan (5). All these facts indicate the expanding importance of L. garvieae.Despite the role of this microorganism as an infectious agent, few studies of the epidemiology of L. garvieae recovered from fish have been published until now. Epidemiological investigations depend on the availability and reliability of highly discriminatory typing systems which may differentiate between strains from different sources (23). Ribotyping (RT) and pulsed-field gel electrophoresis (PFGE) have been used for epidemiological characterization of L. garvieae and have shown high genetic variability within this species (12, 35). The typing schemes proposed by these authors are complicated and have limited epidemiological value. Moreover, the RT and PFGE techniques usually involve time-consuming steps and specific equipment (16,25). Randomly amplified polymorphic DNA (RAPD) is an accesible and sensitive method based on the use of arbitrary primers to amplify polymorphic segments of DNA (38). Although this technique has been widely used in recent years for the study of genetic diversity among isolates of a number of bacterial fish pathogens (3,18,24,27,30,33,34,37) to our knowledge no studies were reported for L. garvieae.In the p...