diff --git a/esp_rfc2217_server.py b/esp_rfc2217_server.py
index 8b9a167..cf83a18 100755
--- a/esp_rfc2217_server.py
+++ b/esp_rfc2217_server.py
@@ -256,7 +256,7 @@ def main():
     logging.getLogger("rfc2217").setLevel(level)
 
     # connect to serial port
-    ser = serial.serial_for_url(args.SERIALPORT, do_not_open=True)
+    ser = serial.serial_for_url(args.SERIALPORT, do_not_open=True, exclusive=True)
     ser.timeout = 3  # required so that the reader thread can exit
     # reset control line as no _remote_ "terminal" has been connected yet
     ser.dtr = False
diff --git a/esptool/loader.py b/esptool/loader.py
index 802531f..56aab58 100644
--- a/esptool/loader.py
+++ b/esptool/loader.py
@@ -300,7 +300,7 @@ class ESPLoader(object):
 
         if isinstance(port, str):
             try:
-                self._port = serial.serial_for_url(port)
+                self._port = serial.serial_for_url(port, exclusive=True)
             except serial.serialutil.SerialException as e:
                 port_issues = [
                     [  # does not exist error
diff --git a/setup.py b/setup.py
index 67912bc..39d88e0 100644
--- a/setup.py
+++ b/setup.py
@@ -128,7 +128,7 @@ setup(
         "bitstring>=3.1.6",
         "cryptography>=2.1.4",
         "ecdsa>=0.16.0",
-        "pyserial>=3.0",
+        "pyserial>=3.3",
         "reedsolo>=1.5.3,<1.8",
         "PyYAML>=5.1",
         "intelhex",