Two isoquinoline alkaloids, including a new natural product (1) and a compound (2) isolated from plant for the first time, and named fistulatins A and B (1 and 2), together with four known isoquinoline alkaloids (3-6) were isolated from the twigs of Cassia fistula. Their structures were determined by means of HRESIMS and extensive 1D and 2D NMR spectroscopic studies. Compounds 1-6 were tested for their anti-tobacco mosaic virus (anti-TMV) activity. The results showed that all isolates showed weak anti-TMV activity with inhibition rates in the range of 15.4-23.5%. © 2016 The Japan Institute of Heterocyclic Chemistry.