Colibacillosis is an infectious disease caused by Escherichia coli in ruminants and poultry. Marine fungi contain bioactive and antibacterial compounds. The aim of this study was to assess the antibacterial activity of extracts of endophytic marine fungal isolates associated with mangrove Aspergillus terreus (WB 1-2) to inhibit the growth of the pathogenic bacteria E. coli. The research was carried out using several stages of fungal cultivation, extraction of active compounds, and in vitro tests of antibacterial activity. Extraction was performed by the maceration method using ethyl acetate and concentrated with a rotary evaporator. Testing of antibacterial activity using the paper disc diffusion method. The measured parameter was the magnitude of the inhibitory diameter formed around the disc paper. The concentrations of the samples used were 0.5, 1, and 2 mg. Ethyl acetate (sample solvent) was used as a negative control, and chloramphenicol was used as a positive control. The results showed that extract of Aspergillus terreus (WB 1-2) showed the highest inhibition zone at a 2 mg/µL concentration of 17.60±5.35 mm was categorised as strong and had bacteriocidal activity.