The motion of single bubbles rising in a highly viscous hydrophobically modified alkali-soluble emulsion (HASE) polymer solution is experimentally examined. In the experiment, a 1.6 wt% HASE polymer solution, which is adjusted to pH ≈ 7.0 by adding a sodium hydroxide solution, is used. In this study, we focus on the long threadlike shapes formed at the bottom of a bubble. Our observations are recorded using two high-speed video cameras. The morphology of trailing edges with threadlike branches is sensitive to the bubble size. For increasing bubble size, the number of threadlike branches observed also increases; more than 10 threadlike-branches are observed for large sized bubbles. Besides reporting on bubble morphology for single bubbles rising in a highly viscous HASE material, we also report on a discontinuity change in the relation of bubble rise velocity vs bubble volume.