(p, p, T) measurements and visual observations of the meniscus for ammonia were carried out carefully in the critical region over the range of temperatures: -1 K <= (T - T-c) <= 0.04 K, and of densities: -19 kg m(-3) <= (p - p(c)) <= 19 kg.m(-3) by a metal-bellows volumometer with an optical cell. Vapor pressures were also measured at T= (310, 350, and 400) K. The critical parameters of T-c and p(c) were determined based on the results of observation of the critical opalescence. The critical pressure p(c) was determined from the present measurements at T-c on the vapor pressure curve. Comparisons of the critical parameters with values given in the literature are presented. (c) 2005 Elsevier Ltd. All rights reserved.