diff --git a/barretenberg/rust/barretenberg-rs/src/types.rs b/barretenberg/rust/barretenberg-rs/src/types.rs
index bb6fd3a0c3af..6ad04b26ecd7 100644
--- a/barretenberg/rust/barretenberg-rs/src/types.rs
+++ b/barretenberg/rust/barretenberg-rs/src/types.rs
@@ -7,10 +7,10 @@ use serde::{Deserialize, Serialize};
 pub struct Fr(pub [u8; 32]);
 
 impl Fr {
-    /// Create a new field element from a u64 value (little-endian encoding)
+    /// Create a new field element from a u64 value (big-endian encoding, matching C++ msgpack format)
     pub fn from_u64(value: u64) -> Self {
         let mut bytes = [0u8; 32];
-        bytes[0..8].copy_from_slice(&value.to_le_bytes());
+        bytes[24..32].copy_from_slice(&value.to_be_bytes());
         Fr(bytes)
     }