The aim of the present study was to assess whether the critical speed calculated by the slope of the distance–time relationship (S d–t ) represents the boundary between the heavy and severe intensity domains in swimming and would be sustainable during intermittent exercise. Nine competitive male swimmers (mean±SD: aged 21.2±2.6 yrs; peak V˙O2 of 3866±529mLmin −1 ) performed, (a) four fixed-distance (100–200–400–800m) all-out efforts to determine S d–t and peak V˙O2; (b) three constant-speed efforts to exhaustion (TTE) at and 5% above and below S d–t (Sd–t+5% and Sd–t−5%, respectively); (c) a set of 10×400m at S d–t with 40-s recovery in between. Capillary blood lactate concentration ([La] B ), oxygen uptake (V˙O2), and RPE remained stable at Sd–t−5% (TTE=48.9±14.1min) with end values of 3.8±1.9mmolL −1 , 87±14% peak V˙O2, and 4.7±1.3. TTE decreased at Sd–t+5% (8.6±3.1min), with end [La] B of 10.2±1.9mmolL −1 . Peak V˙O2 was reached at exhaustion. Similarly, S d–t could only be maintained for 24.3±7.7min with an increase in RPE and [La] B , V˙O2 reaching its peak (95±5% peak V˙O2). RPE increased but [La] B remained stable throughout the ten 400m blocks performed at S d–t (overall time of 53.9±2.7min). The physiological responses when swimming 5% below and 5% above S d–t are those characterising the heavy and severe intensity domain, respectively. While S d–t lies within the severe intensity domain, intermittent swims at this intensity induce [La] B steady state alongside high rates of perceived exertion.