Hot temperatures negatively impact global livestock production and are a serious financial burden to pig producers. Trends predict that global temperatures will continue toincrease, making this a pertinent issue now and in the future. Study objectives were to investigate the effects of genetics and dietary fiber content on growth,feed intake, and feed conversion efficiencyin response to repeated exposure to heat stress.Atotal of 103 finishing barrows from a contemporary commercial line and Yorkshire linesdivergently selected for high and low feed efficiency based on residual feed intake (RFI) were used in this experiment. All pigs were subjected three consecutivetimes to a 4-d heat stress (HS) load, starting with a 19-d thermal neutral (TN) adaptation period and alternated by 7-d TN conditions(see Figure 1). Feed intake, growth and feed conversion efficiency were measured for each period. Heat stress negatively impacted all three production traits, however, pigs from the commercial line were most affected. In addition,during heat stress,pigs from the low RFI (= more efficient) line lost their advantage in feed conversion efficiency over pigs from the high RFIline.This supports the hypothesis that pigs with a high genetic propensity for lean tissue accretion are more susceptible to heat stress. However, our results also indicatedthat the effect ofheat stress onfeed intake, growth rate, and feed conversion efficiency was not significantly influenced by theamount of fiberin the diet (at constant energy density).