There were five new diterpenoids, 18-beta-D-3',4'-diacetoxyxylopyranosyl-ent-kaur-16-ene (1), 18-beta-L-3',5'-diacetoxyarabinofuranosyl-ent-kaur-16-ene (2), 18-beta-D-3',6'-diacetoxyglucopyranosyl-ent-kaur-16-ene (3), ent-isopimar-8(14),15-dien-19-oic acid (4), and 5alpha-hydroxy-ent-rosa-15-en-18-oic acid (5), isolated from the whole herb of Sagittaria pygmaea. Their structures and relative configurations were established based on spectroscopic studies, chemical methods, and X-ray crystallographic analysis. Compound 2 exhibited significant antibacterial activity against the oral pathogens, Streptococcus mutans ATCC 25 175 and Actinomyces viscosus ATCC 27 044, with MIC values against both pathogens of 15.6 microg/mL. Compound 3 was active against only A. viscosus ATCC 27 044 with an MIC value of 62.5 microg/mL. Compounds 4 and 5 were active against S. mutans ATCC 25 175 and A. viscosus ATCC 27 044, with MIC values against both pathogens of 125.0 microg/mL.