To investigate how stem anatomical structure is linked to growth and resistance to stem-boring insects in a herbaceous species, six populations of alligatorweed (Alternanthera philoxeroides) were grown in a common garden. Stem growth rate (GR) of A. philoxeroides and pupation rate as an estimate of resistance to a stem-boring insect (Agasicles hygrophila) were quantified. Stem tissue mass density (TMD) was measured and stem anatomical traits were analysed on cross-sectional areas (CSA). Stem TMD was positively correlated with resistance (i.e. negatively correlated with pupation rate) and negatively correlated with GR. Stem cortex CSA (%) and vascular bundle (VB) density (no./mm2) were positively related to stem TMD and negatively related to pupation rate. The GR was positively related to VB CSA (%) and negatively related to VB density. These results suggest that stem TMD, which results from a high fraction in cortex CSA and high VB density, is a key determinant of resistance to a stem-boring specialist in A. philoxeroides. The high resistance of plants with higher stem TMD may partially impose a cost to plant growth.