diff --git a/common/eth2_network_config/built_in_network_configs/mainnet/config.yaml b/common/eth2_network_config/built_in_network_configs/mainnet/config.yaml
index 47b02aa8d9..8be60242b0 100644
--- a/common/eth2_network_config/built_in_network_configs/mainnet/config.yaml
+++ b/common/eth2_network_config/built_in_network_configs/mainnet/config.yaml
@@ -23,7 +23,7 @@ GENESIS_DELAY: 604800
 
 # Altair
 ALTAIR_FORK_VERSION: 0x01000000
-ALTAIR_FORK_EPOCH: 18446744073709551615
+ALTAIR_FORK_EPOCH: 74240
 # Merge
 MERGE_FORK_VERSION: 0x02000000
 MERGE_FORK_EPOCH: 18446744073709551615
diff --git a/consensus/types/src/chain_spec.rs b/consensus/types/src/chain_spec.rs
index a9e7690326..ee213f1f87 100644
--- a/consensus/types/src/chain_spec.rs
+++ b/consensus/types/src/chain_spec.rs
@@ -349,8 +349,6 @@ impl ChainSpec {
     }
 
     /// Returns a `ChainSpec` compatible with the Ethereum Foundation specification.
-    ///
-    /// Spec v0.12.3
     pub fn mainnet() -> Self {
         Self {
             /*
@@ -468,7 +466,7 @@ impl ChainSpec {
             domain_sync_committee_selection_proof: 8,
             domain_contribution_and_proof: 9,
             altair_fork_version: [0x01, 0x00, 0x00, 0x00],
-            altair_fork_epoch: None,
+            altair_fork_epoch: Some(Epoch::new(74240)),
 
             /*
              * Network specific