This is the dataset for the article entitled: Effects of constant versus fluctuating temperatures on fitness indicators of the aphid Dysaphis plantaginea and the parasitoid Aphidius matricariae. Like all organisms, insects encounter temperatures that fluctuate on different time scales: within a day, between days, or throughout the seasons. However, most studies on the impact of temperature on insect physiology, behaviour, morphology or ecology have focused on constant temperatures tested in the laboratory. In our study, we wanted to know if fluctuating temperatures during the day (7—17°C, average 12°C) can affect insects differently compared to a constant temperature of 12°C. We use as a model the apple aphid Dysaphis plantaginea, a major threat to apple orchards worldwide, and its parasitoid Aphidius matricariae, which is used in biological control. We found that many traits—but not all—were affected. In particular, the fluctuating thermal regime decreased the development time of aphids and parasitoids, improved the rate of parasitism, and tended (albeit slightly) to improve the survival of both species. In contrast, we did not find strong effects on morphological traits. Our results can be used to better predict how these agronomically important insects behave in orchards, how fluctuating temperatures affect host-parasitoid relationships, and ultimately what the implications are in the context of climate change and biological control.