An original in situ ultrasonic echography technique was used to study the thermal stability and crystallisation of a Zr55Cu30Al10Ni5 bulk metallic glass between RT and 630 degreesC. Changes in Young's modulus with temperature were reported allowing to study the supercooled-liquid state and the crystallisation process. Investigations of viscoclastic properties gave information on the correlation factor (hierarchically correlated motion theory) and three distinct crystallisation stages were observed. Their kinetics were studied using Voigt's and Reuss' approximations for a two-phase material and comparisons with the Johnson-Mehl-Avrami-Kolmogorov theory allowed us to consider a mixed surface/internal nucleation for the first stage and a surface nucleation for the two last stages. (C) 2002 Elsevier Science Ltd. All rights reserved.