Kinetic instabilities, notably the whistler heat-flux instability (WHFI), are known to suppress thermal transport significantly in the moderate- to high-
beta
β
$\beta$
plasmas relevant to many astrophysical systems. This paper explores WHFI-regulated heat transport in a new regime: ultra-high-
beta
β
$\beta$
plasmas with
beta Subscript e Baseline greater than or equivalent to upper L Subscript normal upper T Baseline divided by rho Subscript e
β
e
≳
L
T
/
ρ
e
$\beta _{e} \gtrsim L_{\mathrm{T}}/\rho _e$
(where
beta Subscript e
β
e
$\beta _e$
is the ratio of electron thermal to magnetic pressure,
rho Subscript e
ρ
e
$\rho _{e}$
is the electron Larmor radius and
upper L Subscript normal upper T
L
T
$L_{\mathrm{T}}$
is the macroscopic temperature-gradient length scale). Extrapolating previous theories of the WHFI to ultra-high-
beta
β
$\beta$
plasmas, we propose that the magnetic energy in unstable whistler fluctuations becomes comparable to that of the background magnetic field at saturation. We corroborate this hypothesis with kinetic simulations using the particle-in-cell code OSIRIS in one spatial dimension and three velocity dimensions (1D3V) and two spatial dimensions and three velocity dimensions (2D3V). We also find that, in ultra-high-
beta
β
$\beta$
plasmas, the heat flux is localised and no longer regulated primarily by resonant pitch-angle scattering of electrons; instead, thermal energy is transported predominantly by advection at the phase velocity of energetically dominant whistlers. Heat-flux suppression is observed in both 1D3V and 2D3V simulations; however, we show that the saturation of the WHFI and the regulation of heat flux are sensitive to dimensionality in the ultra-high-
beta
β
$\beta$
regime. Both the amplitude and phase velocity of the heat-flux-regulating whistler waves scale differently with
beta Subscript e
β
e
$\beta _e$
, yielding parallel heat fluxes, normalised to the free-streaming value, of
q Subscript e parallel to Baseline divided by q Subscript normal f normal s Baseline almost equals 4.7 beta Subscript e Superscript negative 1
q
e
∥
/
q
f
s
≈
4.7
β
e
−
1
$q_{e\parallel } / q_{\mathrm{fs}} \approx 4.7 \beta _{e}^{-1}$
and
q Subscript e parallel to Baseline divided by q Subscript normal f normal s Baseline almost equals 0.3 beta Subscript e Superscript negative 1 divided by 2
q
e
∥
/
q
f
s
≈
0.3
β
e
−
1
/
2
$q_{e\parallel } / q_{\mathrm{fs}} \approx 0.3 \beta _{e}^{-1/2}$
in 2D3V and 1D3V simulations, respectively. We also perform 2D3V simulations with background magnetic fields inclined to the temperature gradient, showing that perpendicular heat transport remains negligible in ultra-high-
beta
β
$\beta$
plasmas; this indicates that anomalous diffusive processes contribute negligibly to cross-field transport. We develop an heuristic theory from kinetic equations that explains these phenomena. Our work extends our understanding of how the WHFI modifies thermal transport to regimes applicable to high-energy-density physics and the reionised intergalactic medium.