Podolactones A and B, which strongly inhibit expansion and mitosis of plant cells, are shown to be norditerpene dilactones with structures (I) and (II). A partially purified extract of the bark of Podocarpus species (cf. P. neriifolim) from Northern Queensland was found to inhibit strongly cell expansion in the pea-segment system described by Adamson et al. Further fractionation afforded two compounds, podolactones A and B, with activities equal to or greater than that of abscisic acid in the above system. Molecular formulae were established by microanalysis: podolactone A (m.p. 291--293°C, decomp.), C₁₉H₂₂O₇ and podolactone B (m.p. 272-275°C, decomp.) C₁₉H₂₂O₈. Both compounds have UV absorption (ethanol) at 218nm (ε 12,500) and IR bands (KBr) near 1715 and 1640cm⁻¹, suggesting the presence of an α,β-unsaturated δ-lactone, while bands near 1775 and 3500cm⁻¹ indicate γ-lactone and hydroxy-groups. These data are closely similar to those for inumakilactone (III), a bisnorditerpenoid dilactone isolated from Podocarpus macrophyllus. NMR data further confirm the similarity and indicate the structures. The structure (II) for podolactone B was confirmed by acetylation to the 3,16-diacetate (IV), selective hydrolysis to the 3-acetate (V), and oxidation to a methyl ketone matching that from inumakilactone 3-acetate. Podolactone A, which has one oxygen less than B and no hydroxy-group at C-3, was assigned structure (I). Mass spectra show base peaks at m/e 304 (A, M-74) and 320 (B) from McLafferty rearrangement. The podolactones strongly inhibited growth of hook and apical segments of pea stems at 2.5×10⁻⁵M: after 24hr, the increase in weight of hook segments as a percentage of control was 25% (A) and 29% (B), compared to 53% for (±)-abscisic acid (2×10⁻⁵M). Podolactone A also acted as an inhibitor of cell division in the Jerusalem artichoke system: freshly cut slices treated with auxin, cytokinin, and calcium chloride showed a mitotic frequency of 5.7% after 37hr and 7.0% after 50hr, while addition of podolactone A (2.5×10⁻⁵M) prevented mitosis until 50hr, when the mitotic frequency was only 2.4%.