Transmission studies demonstrated that canine brucellosis can spread from infected to susceptible males maintained in close contact after 4 to 6 months of cohabitation. Spread by males occurred after epididymitis was observed in the infected dogs. Transmission via contaminated urine was suspected, but not proved. The bladder urine of infected males, probably contaminated with seminal fluid, contained higher numbers of B. canis organisms than did that of female dogs. Highest concentrations of bacteria in urine were found between postinfection weeks 8 and 12. Infected females transmitted the infection to contact females after 5 months. Immature females or males infected with B. canis did not transmit brucellosis until after an estrus or a mating was observed--about post-contact months 10-12.