Abstract Background The most used protein sources in ruminant nutrition are considered as having negative impacts in terms of environmental sustainability and competition with human nutrition. Therefore, the investigation of alternative and sustainable feedstuffs is becoming a priority in ruminant production systems. Results This trial was designed to evaluate eight full-fat insect meals ( Acheta domesticus – ACD; Alphitobius diaperinus – ALD; Blatta lateralis – BL; Gryllus bimaculatus – GB; Grylloides sygillatus – GS; Hermetia illucens – HI; Musca domestica – MD; and Tenebrio molitor – TM) as potential protein and lipid sources in ruminant nutrition. Fermentation parameters and fatty acids (FA) of rumen digesta after 24-h in vitro ruminal incubation of the tested insect meals were measured and compared with those of three plant-based meals (soybean meal, rapeseed meal and sunflower meal) and fishmeal (FM). Similarly to FM, the insect meals led to a significantly lower total gas production (on average, 1.75 vs. 4.64 mmol/g dry matter—DM), methane production (on average, 0.33 vs. 0.91 mmol/g DM), volatile FA production (on average, 4.12 vs. 7.53 mmol/g DM), and in vitro organic matter disappearance (on average, 0.32 vs. 0.59 g/g) than those observed for the plant meals. The insect meals also led to lower ammonia of rumen fluid, when expressed as a proportion of total N (on average, 0.74 vs. 0.52 for the plant and insect meals, respectively), which could be an advantage provided that intestinal digestibility is high. Differences in ruminal fermentation parameters between the insect meals could be partially explained by their chitin, crude protein and ether extract contents, as well as by their FA profile. In particular, high content of polyunsaturated FA, or C12:0 (in HI), seems to partially inhibit the ruminal fermentations. Conclusions The tested full-fat insect meals appear to be potentially an interesting protein and lipid source for ruminants, alternative to the less sustainable and commonly used ones of plant origin. The FA profile of the rumen digesta of ACD, ALD, GB, GS and TM, being rich in n-6 polyunsaturated FA, could be interesting to improve the quality of ruminant-derived food products.