Analysis of the 250-hPa divergence anomalies and their role in the onset of atmospheric blocking in the Southern Hemisphere derived from the NCEP-NCAR reanalysis data during 1948-1999 have been investigated. Blocking episodes were defined by an objective blocking index and 30 cases were studied, 10 during each of three phases of ENSO (warm, cold, neutral). The full divergence tendency equation was utilized to diagnose the local development of divergence anomalies prior to the onset of blocking. Results indicate that in at least some of the cases, particularly during warm and cold phases of ENSO, the temporal increase of divergence near the block-onset region was forced primarily by horizontal advection and by anticyclonic ageostrophic relative vorticity. The contributions by other processes will be studied and reported. Time series of blocking frequency revealed two regions of preferred occurrence of blocking in the South Pacific, one between 140E-160E and the other between 120W-140W. Mean divergence anomalies during blocking were greater for the cold and warm phases of ENSO and also greater in the South-Eastern Pacific region. Monthly relative frequencies of total blocked days and blocked days for each phase of ENSO indicated a shift toward the late austral fall and early winter during the warm phase, whereas in the case of the cold and neutral phases the highest frequency was in June-July