The effects of manufactured carbon black (CB) nanoparticles on reproduction and early development of the marine brown algae Fucus serratus were studied. Results obtained indicated that effects of CB on sperm concentration and fertilization, body axis alignment, germination and rhizoid elongation were primarily physical. CB nanoparticles added to sperm suspension and embryo cultures, formed large agglomerates that removed sperm from suspension and reduced fertilization success at 100 mg l(-1). Correct alignment of the polar axis was reduced at 50 mg l(-1). From EM imaging there was no clear evidence of CB nanoparticle uptake in the zygotes. Instead zygotes were covered by agglomerates of CB nanoparticles that may have shaded for incident light that is crucial for alignment of the polar body axis. Germination and rhizoid elongation, both independent of light, were unaffected by CB nanoparticles.