Pulsed-field gel electrophoresis (PFGE) was used to investigate the diversity of Streptococcus suis isolates of various serotypes recovered from swine clinical samples in Spain. Capsular types 9 (64.9%) and 2 (14.8%) were the most frequently isolated serotypes followed by serotype 7 (5.9%) and serotype 8 (4.3%). The PFGE results of this study with 60 different pulsotypes indicate a great genetic diversity among the S. suis isolates, which is consistent with the broad distribution of S. suis in the swine population. Forty-five percent of the pulsotypes corresponded to single isolates, no pulsotype was common to all farms, and at least 3 different pulsotypes were isolated in 56% of herds in which more than 3 clinical isolates were analyzed. These results reveal a great diversity both between and within herds throughout the strains of S. suis studied, demonstrating that different strains of S. suis are associated with infection in pigs. Some pulsotypes were more frequently isolated and exhibited a wider distribution over herds than others, and were the unique or predominant strains in several herds, suggesting the existence of a prevalent or a few prevalent clones responsible for a large proportion of clinical cases. Overall, the great genetic heterogeneity of the clinical strains of S. suis, the isolation of different strains within the same herd, and the predominance of particular strains in some herds are evidence that infection by S. suis is a dynamic process and reinforce the idea that the epidemiology of S. suis infection is very complex.