AbstractIn its area of distribution, the heartwood of Eysenhardtia polystachya (palo dulce) is empirically known to be very durable. To verify the reputed high natural durability, its resistance to wood degrading fungi was tested by means of the soil block and agar block tests using two white rot (Trametes versicolor and Phanerochaete chrysosporium) and two brown rot (Postia placenta and Gloeophyllum trabeum) fungi. Recorded mass losses for the soil block test (between 1·3 and 3·8%) were significantly lower than for the agar block test (between 7 and 10%). The higher mass loss in the agar block test is believed to be partly due to migration of extractives out of the blocks and not due to loss in mass as a result of fungal decay. According to the numerical values of mass loss recorded for the two test series, E. polystachya heartwood is rated 'highly resistant' (class 1 according to ASTM D 2017-05 – soil block test) and 'durable' (class 2 according to EN 350-1 – agar block test).Keywords: EYSENHARDTIA POLYSTACHYANATURAL DURABILITYSOIL AND AGAR BLOCK TESTWHITE ROT AND BROWN ROT FUNGI