By a Monte Carlo computation we have estimated the spectrum of x‐rays scattered into the eclipse of 4U1538‐52 by the atmosphere of its supergiant companion. The estimate fails to account for a soft component below 4.5 keV which amounts to approximately 1.4% of the average uneclipsed flux in the same energy range. The intensity of the soft component exhibits an initial downward trend following eclipse ingress as expected of a component scattered by interstellar dust grains. Comparing the intensity with the optical extinction of QV Nor, we derive an upper limit on a quantityRXV(E) which we call the scattering/extinction ratio of interstellar dust grains and define as (E/1 keV)2times the ratio of optical depth for scattering x‐rays of energyEto the total optical extinction. In the Rayleigh‐Gans approximation to the x‐ray scattering efficiency, this quantity is independent of energy. Our upper limit onRXVis 0.06 mag−1, which implies that the x‐ray scattering efficiency of interstellar dust is less than expected for solid grains with a size distribution of the formng(a)∼a−3.5in the range from 0.005 &mgr;m to 0.25 &mgr;m and composed of silicate (RXV=0.22 mag−1) or a silicate‐graphite mixture (RXV=0.11 mag−1) as derived from the calculations of Martin Rouleau (1991). This lends support to the idea (Mathis & Whiffen 1989) that interstellar grains are ‘‘fluffy’’ aggregates with an average bulk density less than that of their constituent particles. Such aggregates would have a smaller ratio of x‐ray scattering efficiency to optical extinction efficiency compared to solid grains of the same material. ©1994 American Institute of Physics.