The swimming patterns of zoospores of the green alga Ulva linza in the vicinity of a surface were investigated by digital in-line holography. Full 3D motion patterns were retrieved from measurements and the traces obtained were compared with known swimming patterns of spores of the brown alga Hincksia irregularis and the green alga Ulva linza as seen in a conventional optical microscope. Quantitative information such as swimming velocity was calculated from the 3D traces. The results demonstrate the potential of digital in- line holography to image and quantify exploratory patterns of behavior of motile spores close to surfaces. This technique can give detailed insight into mechanisms of surface colonization by spores and larvae of fouling organisms in response to changes in surface properties.