diff --git a/CHANGELOG.md b/CHANGELOG.md
index 13354d6..7631ac0 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,6 +1,13 @@
 # Change Log
 ### All notable changes to `AMICO` will be documented in this file.
 
+## [Unreleased]
+### 🐛Fixed
+- Removed `isExvivo` parameter from CylinderZeppelinBall model
+
+---
+---
+
 ## `v2.1.1`<br>_2025-08-13_
 ### 🐛Fixed
 - `UserWarning`: `pkg_resources` is deprecated as an API.
diff --git a/amico/models.pyx b/amico/models.pyx
index d7340c6..6bfde76 100644
--- a/amico/models.pyx
+++ b/amico/models.pyx
@@ -432,7 +432,6 @@ class CylinderZeppelinBall( BaseModel ) :
         params['Rs'] = self.Rs
         params['d_perps'] = self.d_perps
         params['d_isos'] = self.d_isos
-        params['isExvivo'] = self.isExvivo
         return params
 
 
@@ -546,16 +545,12 @@ class CylinderZeppelinBall( BaseModel ) :
     def _fit(self, thread_id, y, dirs, hash_table, kernels):
         # configs
         cdef int thread_id_c = thread_id
-        cdef bint is_exvivo = 1 if self.isExvivo else 0
         cdef bint compute_rmse = 1 if self.configs['compute_rmse'] else 0
         cdef bint compute_nrmse = 1 if self.configs['compute_nrmse'] else 0
         cdef int n_rs = len(self.Rs)
         cdef int n_perp = len(self.d_perps)
         cdef int n_iso = len(self.d_isos)
         cdef int n_atoms = n_rs + n_perp + n_iso
-        # NOTE not implemented
-        if is_exvivo:
-            n_atoms += 1
 
         # solver params
         cdef double lambda1 = self.solver_params['lambda1']