Eighteen new nor-isopimarane diterpenes, xylarinorditerpenes A-R (<b>1</b>-<b>18</b>), along with two previously reported compounds, 14α,16-epoxy-18-norisopimar-7-en-4α-ol (<b>19</b>) and the labdane-type diterpene agatadiol (<b>20</b>), were isolated from cultures of the fungicolous fungus <i>Xylaria longipes</i> HFG1018 isolated from the wood-rotting basidiomycete <i>Fomitopsis betulinus</i>. The structure elucidation and relative configuration assignments of <b>1</b>-<b>18</b> were accomplished by interpretation of spectroscopic data and through computational methods. The absolute configurations of <b>1</b>, <b>4</b>, and <b>16</b> were determined by single-crystal X-ray diffraction. Compounds <b>1</b>-<b>16</b> possess an 18- or 19-nor-isopimarane skeleton, and compounds <b>17</b> and <b>18</b> possess an 18,19-dinor-isopimarane skeleton. Compounds <b>2</b>-<b>5</b>, <b>9</b>, <b>14</b>, <b>19</b>, and <b>20</b> showed immunosuppressive activity but were devoid of cytotoxicity against the cell proliferation by concanavalin A-induced T lymphocytes and lipopolysaccharide-induced B lymphocytes, with IC<sub>50</sub> values varying from 1.0 to 27.2 μM and from 16.1 to 51.8 μM, respectively.