Paradiplosis tumifexinduces a simple, single-chambered, prosoplasmic gall on the adaxial surface of current-year needles of balsam fir. Proliferating and enlarged mesophyll cells surround the immature larva except for an ostiolar opening on the adaxial surface. The vascular bundle is not affected by gall formation, but the cells lining the resin ducts are altered. As the gall matures the epidermis and one or two layers of underlying mesophyll cells become lignified. Concentrations of starch granules are retained in gall cells after starch has been dissipated in tissues beyond the gall and in nongalled needles. Host damage occurs when galled needles dry and abscise prematurely.