Bioluminescence is widespread in the deep sea. In this study, the density of bioluminescent zooplankton (BL) in the deep Mediterranean Sea (MS) and at the Mid- Atlantic Ridge (MAR) was measured using low light video cameras, including a novel Image intensified Charge coupled device for Deep-sea research (ICDeep). Sampling across the MS was undertaken to assess the potential optical interference from bioluminescent zooplankton at sites under consideration for the construction of a large volume deep-sea neutrino telescope. This revealed a general decrease in deep pelagic BL densities from the western to the eastern MS. Deep mesopelagic (500- 1000 m depth) BL density was significantly correlated (p<O.Ol) to Chlorophyll a concentrations in overlying surface waters. Investigation of temporal variation across the Ionian Sea revealed a deep (500-2400 m depth) peak in BL density on the western side, present in the autumn (mean BL density: 14.92 m-3) and absent the following spring (4.52 rrr"). Deep water of Adriatic origin flows into this region and may seasonally affect the supply of organic material, increasing deep zooplankton populations. Near-bed (0-400 m above bed, mab) BL density in the MS was found to decrease exponentially with seafloor depth; by a factor of 4.2 every 1000 m. BL densities were consistently lower in the MS than at the MAR. On the MAR, at ca. 2500 m depth, an investigation of altitude effects within the near seafloor region revealed a weak increase (0.016 m-3 mab': p<O.OOl) in BL and inacroparticle (>430 urn] densities from 100 to 5 mab attwo sites south of the Charlie-Gibbs Fracture Zone. At 2500 m depth on the MAR, a rate of naturally occurring bioluminescent events was observed to be 155 times higher (3.1 min+) than literature based predictions, prompting a reappraisal of the visual environment in relation to complex topography of the deep-sea floor.