Experimental data collected in recent years by Crombie [1958], and Reder et al. [1962], and theoretical work on VLF propagation by Wait [1960] have given strong evidence to the fact that VLF propagation is nonreciprocal in the east-w'es t and west-east direc Lioos. Experimental data rela ting to this topic are aL present sparse. This paper presents some experjmentflJ results obtained during the past 2 years which will substantiate the existing evidence as to this nonreciprocal propagation . Since D ecemb er 1961 , t he phftse of the VLF transmitter NPM (19.8 kc/s) located at Hawaii has been monitored and compar ed with a local high stability frequency source in Singapore, which is approximately 11,000 km away. The shortest propagation pftth is one which travels within 20 deg of an east to wes t direcLion. Th e relfttive phftse of GBR (distance 11,000 km) has also been monitored over the same period. While tbe diurnal variation of phase of GBR (16.0 kc/s) at t his iLe has undergone tbe usual trapezoidal pattern wiLh a (1fty to night excur ion of 1.1 to 1.3 cycles of phase, tbe diurnal phase of NPM has exhibited a double-humped paLLem regularly day after day (fig . 1). H is proposed in this paper that this form of diurnal phase variation is du e to the nonreciproci ty of VLF propagfttion in the east-west and west-eas t directions. Further evidence to support this conclusion has been obtained from: (a) short term monitoring of NPM at Gan in the M aldive I slands, and (b) from flight measurements of several thousand