Two new linear tetrapeptides, citronamides A (1) and B (2), were isolated from the Australian sponge Citronia astra and their structures determined by 1D and 2D NMR spectroscopy. The peptides contain two previously unreported amino acids, 3-(2-oxo-2,3-dihydro-1H-imidazol-4-yl)serine and 6-amino-7-(4-methoxyphenyl)heptanoic acid, and alpha-iduronic acid modified at either C-3 or C-4 with a carbamate. Citronamide A showed moderate antifungal activity against Saccharomyces cerevisiae.