IT WAS STUDIED THE VARIABILITY IN MORPHOLOGICAL, PHYSICAL AND THERMAL PROPERTIES OF FIBER BARK OF EUCALYPTUS NITENS (DEANE & MAIDEN) COMING FROM TWENTY FOUR TREES, TWO SITES AND SIX FAMILIES; USING TECHNIQUES SUCH AS CONVENTIONAL LIGHT MICROSCOPY, ELECTRON MICROSCOPY (SEM) AND THERMO-GRAVIMETRIC ANALYSIS (TGA). SUCH PROPERTIES ARE RELEVANT FOR A LIGNOCELLULOSIC FIBER TO HAVE A POSSIBLE INDUSTRIAL APPLICATION AS REINFORCEMENT MATERIAL IN A THERMOPLASTIC MATRIX. SIGNIFICANT STATISTICAL DIFFERENCES WERE DETECTED IN THE FIBER PROPERTIES BY ANALYZING THE SITE OF ORIGIN OF THE SAMPLES, FAMILY AND STEM HEIGHT. STANDING OUT IN THE RESULTS OF PHYSICAL PROPERTIES, THE AVERAGE THICKNESS OF THE BARK LAYER (4.4 MM), ITS SPECIFIC GRAVITY (0.757), ITS MORPHOLOGICAL PROPERTIES SUCH AS LENGTH/DIAMETER OF FIBER (LF/DF= 69.14) AND FIBER PRESENCE (45%) IN THIS MATERIAL. THE THERMAL STABILITY OF THIS NATURAL FIBER TO 200ºC AND LOW MELTING TEMPERATURE OF THERMOPLASTICS SUCH AS HIGH DENSITY POLYETHYLENE (HDPE); INDICATE THAT THE BARK FIBERS OF THIS SPECIES ARE A GOOD OPTION TO APPLY AS A REINFORCEMENT MATERIAL IN THERMOPLASTICS MATRICES.