The myctophid genus Diaphus is known for having highly developed head photophores and nasal rosettes. The head photophores of Diaphus are typically enlarged and vary between species and possibly sexes. Diaphus dumerilii is also a macrosmatic species with large olfactory organs. The aim of this study is to quantify the variances in location, size, and shape of the head photophores and nasal rosettes between selected individuals from Diaphus dumerilii to understand how these variances relate to morphometric measurements such as body length, inferred maturity and nasal rosette size and shape. Diaphus possess many highly conserved traits that are useful for species identification, but little is known about the intraspecific characteristics that vary between an individual’s sex, size, and maturity. Comparisons were conducted using computer-aided visualization and image analysis tools and multivariate analyses. From these multivariate analyses it was revealed that the head photophore arrangement and nasal rosette size and shape of Diaphus dumerilii show significant changes with standard length and inferred maturity. However, the photophore arrangement and nasal rosette size and shape did not show significant differences between sexes. By examining how certain D. dumerilii features change with the organism’s size and maturity, it is possible to gain more insight into their function and ecology.