Tilapia samples were collected from five main tilapia farms in Guangdong. The characteristics of the isolates were determined by virulence gene detection

Multilocus Sequencing Type (MLST)

and PulsedField Gel Electrophoresis (PFGE). The frequency of virulence gene in all isolates varied from 60.59% to 100%. Ten distribution patterns of virulence gene were identified

dltR-bca-sodA-spb1-cfb-bac (38.24%

65/170)

dltr-bca-sodA-spb1-bac (23.53%，40/170)

and dltr-bca-sodA-spb1-cfb-bac-scpB (22

35%

38/170) were the predominant patterns

followed by dltr-bca-sodA-spb1-bac-scpb (8.24%

14/170) and dltr-bca-sodA-spb1 (4.71%

8/170). The single allele was observed in each of seven loci

adhP（10）

pheS（1）

atr（2）

glnA（1）

sdhA（3）

glcK（2）

tkt（2）and result in unique ST 7 (Sequencing Type 7). One hundred and fortysix isolates were assigned to six clusters based on the PFGE results. The similarity between these clusters ranged from 86% to 100%. Cluster A (18)

B (16)

C (103) and D (7) were the predominant clusters

account for 98.63% of all isolates. Cluster A and C were distributed in all four areas

Cluster B were mainly isolated from Wuchuan

Maoming and Yangjiang

whereas Cluster D were isolated from Maoming

Zhuhai and Yangjiang. The unique clusters F and G were isolated from Maoming and Zhuhai

respectively. In addition

the clones in Clusters A--D isolated from different areas in Guangdong showed 100% similarity. High proportion of strains with virulence gene was observed in Streptococcus agalactiae strains isolated from tilapia farm in Guandong. All of isolates belonged to the unique ST7. The clones with 100% similarity were identified in different area suggesting the cross infection and transmission in Guangdong

China.