Quantification of growth is an important step to understand the flow of elements and energy in food webs, although in situ estimation of the growth rate is generally very difficult. The present study proposes the nuclear and mitochondrial ribosomal ratio as a novel index of animal growth rate. Mitochondrial ribosomes are responsible for biosynthesis of energy (adenosine triphosphate) producing proteins, while nuclear-encoded cytosolic ribosomes are responsible for other proteins, including cell growth and division. Synthesis of ribosomes requires large amounts of environmentally limiting nutrients such as nitrogen and phosphorus. Therefore, it is hypothesized that the allocation of those elements between the two ribosome types changes with nutrient availability and hence, affects growth rate. The present study conducted laboratory growth experiments of Daphnia magna using various food concentration and temperature treatments. A positive correlation was found between nuclear-encoded cytosolic to mitochondrial ribosomal ratio and somatic growth rate. A significant positive correlation was found between food concentration treatments and the ratio, but not between temperature treatments and the ratio. Our results demonstrate that the ratio between nuclear-encoded cytosolic and mitochondrial ribosomes has the potential to be an effective growth rate estimator, but requires further modification and improvement. As total RNA of community samples is mostly composed of ribosomal RNA, analysis of sequence frequencies using high-throughput sequencers allow application of our findings. Ratios of the obtained ribosomal RNA sequences from the community sample give us not only the taxonomic composition of the community but also a growth index of each group.