The porosity structure of a section of old oceanic crust was characterized by a detailed study of the volcanic sequence of the Troodos Ophiolite. The distribution and abundance of porosity was compared to the amount of secondary minerals in order to calculate how porosity created by crustal accretion is modified by alteration as a section of crust ages. Eight sites with one dominant lithology and contrasting alteration histories were studied. Two types of porosity were quantified: macroscopic (macrofractures, vesicles/vugs, ¿interpillow zones) and total (macroscopic and laboratory-measured microscopic porosity). Crustal accretion produced a vertically zoned porosity structure due to differences between pillows that formed during the main and waning stages of crustal construction. Mean initial macroscopic porosity ranges from 14 to 17% in uppermost pillows and from 6 to 10% in the underlying pillows and massive flows. Macroscopic porosity was reduced by 10--11% where alteration was most pervasive (i.e., ≤250 m of the palcoseafloor). Beneath this zone, pillow and flow outcrops showed decreases of <1--3% and 2--5%, respectively. The combined effects of volcanic morphology and alteration resulted in no systematic variation with depth in the final macroscopic porosity (4--8%). The greatest decrease in macroscopic porosity was localized where abyssal hill topography or volcanic edifices and sedimentation rates facilitated the growth of the seafloor weathering zone, which comprises ~10% of the volcanic pile. Thus in modern ocean basins alteration would have the greatest effect on crustal porosity in areas of rough topography, where crests of abyssal hills remain unsedimented for long periods of time. Porosity data derived from Deep Sea Drilling Project/Ocean Drilling Program cores and borehole logs show similar relationships, which implies that the evolution of upper crustal porosity is controlled by the local environment and that the evolution of upper crustal porosity cannot be uniquely described by seismic data.¿ 1997 American Geophysical Union