Betel leaf extracts have some natural antimicrobial. This extracts has been used as antimicrobial agent in some aqua farmer in Indonesia for an alternative method to control fish diseases. This study aims to determine the effects of betel leaf extract on fish pathogenic bacteria in-vitro. Three kinds of solvents were used in betel leaf extracts. There were ethanol, n-hexane and ethyl-acetate. About 100 g of betel leaf simplicia were diluted by 1000 ml of solvents in stainless container cap for 24 hours maceration. This extracts were filtered with 0.42 μm paper filter and then evaporated at 50C during 30 minutes in 50 rpm with vacuum rotary evaporator. This concentrated extracts were used to antimicrobial testing. The pathogenic bacteria for in-vitro testing were from Gram-negative bacteria such as Aeromonas hydrophila, Vibrio alginolyticus and Edwardsiella tarda. The in-vitro test of herbal extract from betel leaf againts Gram-negative pathogenic bacteria indicated that the betel leaf extract could inhibited the growth of pathogenic bacteria with diameter mean of inhibition zone were more than 14 mm. The solvents variations were not significantly different (p > 0.05) to diameter inhibition zone of V. alginolyticus and A. hydrophila but significantly different (p < 0.05) to E. tarda.